Method, an apparatus and a computer program product for video coding and decoding

ABSTRACT

There are disclosed various methods, apparatuses and computer program products for video encoding and decoding. In some embodiments information on bit-depth of a reference block of an image on a first layer and bit-depth of a current block of an image on a second layer is obtained. The obtained information is used to select one or more resampling filter parameters for filtering at least a part of a reference layer image to be used in inter-layer prediction. There are also disclosed corresponding methods, apparatuses and computer program products for video encoding and decoding.

RELATED APPLICATION

This application was originally filed as PCT Application No. PCT/FI2014/050512 filed Jun. 25, 2014 which claims priority benefit from GB Patent Application No. 1312577.8, filed Jul. 15, 2013.

TECHNICAL FIELD

The present application relates generally to an apparatus, a method and a computer program for video coding and decoding. More particularly, various embodiments relate to scalable and high bit-depth coding and decoding of video.

BACKGROUND

This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.

A video coding system may comprise an encoder that transforms an input video into a compressed representation suited for storage/transmission and a decoder that can uncompress the compressed video representation back into a viewable form. The encoder may discard some information in the original video sequence in order to represent the video in a more compact form, for example, to enable the storage/transmission of the video information at a lower bitrate than otherwise might be needed.

Scalable video coding refers to a coding structure where one bitstream can contain multiple representations of the content at different bitrates, resolutions, frame rates and/or other types of scalability. A scalable bitstream may consist of a base layer providing the lowest quality video available and one or more enhancement layers that enhance the video quality when received and decoded together with the lower layers. In order to improve coding efficiency for the enhancement layers, the coded representation of that layer may depend on the lower layers. Each layer together with all its dependent layers is one representation of the video signal at a certain spatial resolution, temporal resolution, quality level, and/or operation point of other types of scalability.

In the encoding of 3D video content, video compression systems, such as Advanced Video Coding standard (H.264/AVC), the Multiview Video Coding (MVC) extension of H.264/AVC or scalable extensions of HEVC can be used.

SUMMARY

Some embodiments provide a method for encoding and decoding video information. In some embodiments the coding efficiency may be improved and the complexity of a scalable video encoder/decoder may be reduced when both the bit-depth and spatial resolution of base layer picture is increased for enhancement layer. Instead of performing cascaded filtering, a single filtering with reduced number of rounding stages is performed to obtain the inter-layer prediction picture. In some embodiments, the reconstructed base layer image first goes to an upsampling filter where samples of the reconstructed base layer image are multiplied with filter coefficients and a rounding offset may be added to the sums. The rounding offset may depend on both accuracy of filter coefficients and the difference between enhancement and base layer bit-depth. Then, these sums may be shifted to enhancement layer bit-depth. The shift may depend on both accuracy of filter coefficients and the difference between enhancement and base layer bit-depth. A clipping operation may also be performed so that the values of the filtered enhancement-layer image stay within a certain value range.

Various aspects of examples of the invention are provided in the detailed description.

According to a first aspect of the present invention, there is provided a method comprising:

obtaining information on bit-depth of a reference block of an image on a first layer;

obtaining information on bit-depth of a current block of an image on a second layer;

using the obtained information to select one or more resampling filter parameters for filtering at least a part of a reference layer image to be used in inter-layer prediction.

According to a second aspect there is provided an apparatus comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to:

obtain information on bit-depth of a reference block of an image on a first layer;

obtain information on bit-depth of a current block of an image on a second layer;

use the obtained information to select one or more resampling filter parameters for filtering at least a part of a reference layer image to be used in inter-layer prediction.

According to a third aspect there is provided a computer program product including one or more sequences of one or more instructions which, when executed by one or more processors, cause an apparatus to at least perform the following:

obtain information on bit-depth of a reference block of an image on a first layer;

obtain information on bit-depth of a current block of an image on a second layer;

use the obtained information to select one or more resampling filter parameters for filtering at least a part of a reference layer image to be used in inter-layer prediction.

According to a fourth aspect there is provided an apparatus comprising:

means for obtaining information on bit-depth of a reference block of an image on a first layer;

means for obtaining information on bit-depth of a current block of an image on a second layer; and

means for using the obtained information to select one or more resampling filter parameters for filtering at least a part of a reference layer image to be used in inter-layer prediction.

According to a fifth aspect there is provided an apparatus configured to perform the method according to the first aspect.

According to a sixth aspect there is provided a decoder configured to perform the method according to the first aspect.

According to a seventh aspect there is provided an encoder configured to perform the method according to the first aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of example embodiments of the present invention, reference will now be made by way of example to the accompanying drawings in which:

FIG. 1 shows schematically an electronic device employing some embodiments of the invention;

FIG. 2 shows schematically a user equipment suitable for employing some embodiments of the invention;

FIG. 3 further shows schematically electronic devices employing embodiments of the invention connected using wireless and wired network connections;

FIG. 4a shows schematically an encoder suitable for implementing some embodiments of the invention;

FIG. 4b shows schematically an example embodiment of inter-layer processing according to some embodiments of the invention;

FIG. 5a shows schematically a decoder suitable for implementing some embodiments of the invention;

FIG. 5b shows schematically an example embodiment of inter-layer processing block at a decoder;

FIG. 6a illustrates an example of spatial and temporal prediction of a prediction unit;

FIG. 6b illustrates another example of spatial and temporal prediction of a prediction unit;

FIG. 6c depicts an example for direct-mode motion vector inference; and

FIG. 7 shows an example of a picture consisting of two tiles.

DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

In the following, several embodiments of the invention will be described in the context of one video coding arrangement. It is to be noted, however, that the invention is not limited to this particular arrangement. In fact, the different embodiments have applications widely in any environment where improvement of 3D video coding is desired. For example, the invention may be applicable to video coding systems like streaming systems, DVD players, digital television receivers, personal video recorders, systems and computer programs on personal computers, handheld computers and communication devices, as well as network elements such as transcoders and cloud computing arrangements where video data is handled.

The following describes in further detail suitable apparatus and possible mechanisms for carrying out the embodiments. In this regard reference is first made to FIG. 1 which shows a schematic block diagram of an exemplary apparatus or electronic device 50, which may incorporate a codec according to an embodiment of the invention.

The electronic device 50 may for example be a mobile terminal or user equipment of a wireless communication system. However, it would be appreciated that embodiments of the invention may be implemented within any electronic device or apparatus which may require encoding and decoding or encoding or decoding video images.

The apparatus 50 may comprise a housing 30 for incorporating and protecting the device. The apparatus 50 further may comprise a display 32 in the form of a liquid crystal display. In other embodiments of the invention the display may be any suitable display technology suitable to display an image or video. The apparatus 50 may further comprise a keypad 34. In other embodiments of the invention any suitable data or user interface mechanism may be employed. For example the user interface may be implemented as a virtual keyboard or data entry system as part of a touch-sensitive display. The apparatus may comprise a microphone 36 or any suitable audio input which may be a digital or analogue signal input. The apparatus 50 may further comprise an audio output device which in embodiments of the invention may be any one of: an earpiece 38, speaker, or an analogue audio or digital audio output connection. The apparatus 50 may also comprise a battery 40 (or in other embodiments of the invention the device may be powered by any suitable mobile energy device such as solar cell, fuel cell or clockwork generator). The apparatus may further comprise an infrared port 42 for short range line of sight communication to other devices. In other embodiments the apparatus 50 may further comprise any suitable short range communication solution such as for example a Bluetooth wireless connection or a USB/firewire wired connection.

The apparatus 50 may comprise a controller 56 or processor for controlling the apparatus 50. The controller 56 may be connected to memory 58 which in embodiments of the invention may store both data in the form of image and audio data and/or may also store instructions for implementation on the controller 56. The controller 56 may further be connected to codec circuitry 54 suitable for carrying out coding and decoding of audio and/or video data or assisting in coding and decoding carried out by the controller 56.

The apparatus 50 may further comprise a card reader 48 and a smart card 46, for example a UICC and UICC reader for providing user information and being suitable for providing authentication information for authentication and authorization of the user at a network.

The apparatus 50 may comprise radio interface circuitry 52 connected to the controller and suitable for generating wireless communication signals for example for communication with a cellular communications network, a wireless communications system or a wireless local area network. The apparatus 50 may further comprise an antenna 44 connected to the radio interface circuitry 52 for transmitting radio frequency signals generated at the radio interface circuitry 52 to other apparatus(es) and for receiving radio frequency signals from other apparatus(es).

In some embodiments of the invention, the apparatus 50 comprises a camera capable of recording or detecting individual frames which are then passed to the codec 54 or controller for processing. In other embodiments of the invention, the apparatus may receive the video image data for processing from another device prior to transmission and/or storage. In other embodiments of the invention, the apparatus 50 may receive either wirelessly or by a wired connection the image for coding/decoding.

With respect to FIG. 3, an example of a system within which embodiments of the present invention can be utilized is shown. The system 10 comprises multiple communication devices which can communicate through one or more networks. The system 10 may comprise any combination of wired or wireless networks including, but not limited to a wireless cellular telephone network (such as a GSM, UMTS, CDMA network etc), a wireless local area network (WLAN) such as defined by any of the IEEE 802.x standards, a Bluetooth personal area network, an Ethernet local area network, a token ring local area network, a wide area network, and the Internet.

The system 10 may include both wired and wireless communication devices or apparatus 50 suitable for implementing embodiments of the invention.

For example, the system shown in FIG. 3 shows a mobile telephone network 11 and a representation of the internet 28. Connectivity to the internet 28 may include, but is not limited to, long range wireless connections, short range wireless connections, and various wired connections including, but not limited to, telephone lines, cable lines, power lines, and similar communication pathways.

The example communication devices shown in the system 10 may include, but are not limited to, an electronic device or apparatus 50, a combination of a personal digital assistant (PDA) and a mobile telephone 14, a PDA 16, an integrated messaging device (IMD) 18, a desktop computer 20, a notebook computer 22. The apparatus 50 may be stationary or mobile when carried by an individual who is moving. The apparatus 50 may also be located in a mode of transport including, but not limited to, a car, a truck, a taxi, a bus, a train, a boat, an airplane, a bicycle, a motorcycle or any similar suitable mode of transport.

Some or further apparatus may send and receive calls and messages and communicate with service providers through a wireless connection 25 to a base station 24. The base station 24 may be connected to a network server 26 that allows communication between the mobile telephone network 11 and the internet 28. The system may include additional communication devices and communication devices of various types.

The communication devices may communicate using various transmission technologies including, but not limited to, code division multiple access (CDMA), global systems for mobile communications (GSM), universal mobile telecommunications system (UMTS), time divisional multiple access (TDMA), frequency division multiple access (FDMA), transmission control protocol-internet protocol (TCP-IP), short messaging service (SMS), multimedia messaging service (MMS), email, instant messaging service (IMS), Bluetooth, IEEE 802.11 and any similar wireless communication technology. A communications device involved in implementing various embodiments of the present invention may communicate using various media including, but not limited to, radio, infrared, laser, cable connections, and any suitable connection.

Video codec consists of an encoder that transforms the input video into a compressed representation suited for storage/transmission and a decoder that can uncompress the compressed video representation back into a viewable form. Typically encoder discards some information in the original video sequence in order to represent the video in a more compact form (that is, at lower bitrate).

Typical hybrid video codecs, for example ITU-T H.263 and H.264, encode the video information in two phases. Firstly pixel values in a certain picture area (or “block”) are predicted for example by motion compensation means (finding and indicating an area in one of the previously coded video frames that corresponds closely to the block being coded) or by spatial means (using the pixel values around the block to be coded in a specified manner). Secondly the prediction error, i.e. the difference between the predicted block of pixels and the original block of pixels, is coded. This is typically done by transforming the difference in pixel values using a specified transform (e.g. Discrete Cosine Transform (DCT) or a variant of it), quantizing the coefficients and entropy coding the quantized coefficients. By varying the fidelity of the quantization process, encoder can control the balance between the accuracy of the pixel representation (picture quality) and size of the resulting coded video representation (file size or transmission bitrate).

Video coding is typically a two-stage process: First, a prediction of the video signal is generated based on previous coded data. Second, the residual between the predicted signal and the source signal is coded. Inter prediction, which may also be referred to as temporal prediction, motion compensation, or motion-compensated prediction, reduces temporal redundancy. In inter prediction the sources of prediction are previously decoded pictures. Intra prediction utilizes the fact that adjacent pixels within the same picture are likely to be correlated. Intra prediction can be performed in spatial or transform domain, i.e., either sample values or transform coefficients can be predicted. Intra prediction is typically exploited in intra coding, where no inter prediction is applied.

One outcome of the coding procedure is a set of coding parameters, such as motion vectors and quantized transform coefficients. Many parameters can be entropy-coded more efficiently if they are predicted first from spatially or temporally neighboring parameters. For example, a motion vector may be predicted from spatially adjacent motion vectors and only the difference relative to the motion vector predictor may be coded. Prediction of coding parameters within a picture and intra prediction may be collectively referred to as in-picture prediction.

With respect to FIG. 4a , a block diagram of a video encoder suitable for carrying out embodiments of the invention is shown. FIG. 4a presents an encoder for two layers, but it would be appreciated that the presented encoder could be similarly extended to encode more than two layers. FIG. 4a illustrates an embodiment of a video encoder comprising a first encoder section 500 for a base layer and a second encoder section 502 for an enhancement layer. Each of the first encoder section 500 and the second encoder section 502 may comprise similar elements for encoding incoming pictures. The encoder sections 500, 502 may comprise a pixel predictor 302, 402, prediction error encoder 303, 403 and prediction error decoder 304, 404. FIG. 4a also shows an embodiment of the pixel predictor 302, 402 as comprising an inter-predictor 306, 406, an intra-predictor 308, 408, a mode selector 310, 410, a filter 316, 416, and a reference frame memory 318, 418. The pixel predictor 302 of the first encoder section 500 receives 300 base layer images of a video stream to be encoded at both the inter-predictor 306 (which determines the difference between the image and a motion compensated reference frame 318) and the intra-predictor 308 (which determines a prediction for an image block based only on the already processed parts of current frame or picture). The output of both the inter-predictor and the intra-predictor are passed to the mode selector 310. The intra-predictor 308 may have more than one intra-prediction modes. Hence, each mode may perform the intra-prediction and provide the predicted signal to the mode selector 310. The mode selector 310 also receives a copy of the base layer picture 300. Correspondingly, the pixel predictor 402 of the second encoder section 502 receives 400 enhancement layer images of a video stream to be encoded at both the inter-predictor 406 (which determines the difference between the image and a motion compensated reference frame 418) and the intra-predictor 408 (which determines a prediction for an image block based only on the already processed parts of current frame or picture). The output of both the inter-predictor and the intra-predictor are passed to the mode selector 410. The intra-predictor 408 may have more than one intra-prediction modes. Hence, each mode may perform the intra-prediction and provide the predicted signal to the mode selector 410. The mode selector 410 also receives a copy of the enhancement layer picture 400.

The mode selector 310 may use, in the cost evaluator block 382, for example Lagrangian cost functions to choose between coding modes and their parameter values, such as motion vectors, reference indexes, and intra prediction direction, typically on block basis. This kind of cost function may use a weighting factor lambda to tie together the (exact or estimated) image distortion due to lossy coding methods and the (exact or estimated) amount of information that is required to represent the pixel values in an image area: C=D+lambda×R, where C is the Lagrangian cost to be minimized, D is the image distortion (e.g. Mean Squared Error) with the mode and their parameters, and R the number of bits needed to represent the required data to reconstruct the image block in the decoder (e.g. including the amount of data to represent the candidate motion vectors).

Depending on which encoding mode is selected to encode the current block, the output of the inter-predictor 306, 406 or the output of one of the optional intra-predictor modes or the output of a surface encoder within the mode selector is passed to the output of the mode selector 310, 410. The output of the mode selector is passed to a first summing device 321, 421. The first summing device may subtract the output of the pixel predictor 302, 402 from the base layer picture 300/enhancement layer picture 400 to produce a first prediction error signal 320, 420 which is input to the prediction error encoder 303, 403.

The pixel predictor 302, 402 further receives from a preliminary reconstructor 339, 439 the combination of the prediction representation of the image block 312, 412 and the output 338, 438 of the prediction error decoder 304, 404. The preliminary reconstructed image 314, 414 may be passed to the intra-predictor 308, 408 and to a filter 316, 416. The filter 316, 416 receiving the preliminary representation may filter the preliminary representation and output a final reconstructed image 340, 440 which may be saved in a reference frame memory 318, 418. The reference frame memory 318 may be connected to the inter-predictor 306 to be used as the reference image against which a future base layer pictures 300 is compared in inter-prediction operations. Subject to the base layer being selected and indicated to be source for inter-layer sample prediction and/or inter-layer motion information prediction of the enhancement layer according to some embodiments, the reference frame memory 318 may also be connected to the inter-predictor 406 via an inter-layer processing block 450 to be used as the reference image against which a future enhancement layer pictures 400 is compared in inter-prediction operations. Moreover, the reference frame memory 418 may be connected to the inter-predictor 406 to be used as the reference image against which a future enhancement layer pictures 400 is compared in inter-prediction operations.

Filtering parameters from the filter 316 of the first encoder section 500 may be provided to the second encoder section 502 subject to the base layer being selected and indicated to be source for predicting the filtering parameters of the enhancement layer according to some embodiments.

The prediction error encoder 303, 403 comprises a transform unit 342, 442 and a quantizer 344, 444. The transform unit 342, 442 transforms the first prediction error signal 320, 420 to a transform domain. The transform is, for example, the DCT transform. The quantizer 344, 444 quantizes the transform domain signal, e.g. the DCT coefficients, to form quantized coefficients.

The prediction error decoder 304, 404 receives the output from the prediction error encoder 303, 403 and performs the opposite processes of the prediction error encoder 303, 403 to produce a decoded prediction error signal 338, 438 which, when combined with the prediction representation of the image block 312, 412 at the second summing device 339, 439, produces the preliminary reconstructed image 314, 414. The prediction error decoder may be considered to comprise a dequantizer 361, 461, which dequantizes the quantized coefficient values, e.g. DCT coefficients, to reconstruct the transform signal and an inverse transformation unit 363, 463, which performs the inverse transformation to the reconstructed transform signal wherein the output of the inverse transformation unit 363, 463 contains reconstructed block(s). The prediction error decoder may also comprise a block filter which may filter the reconstructed block(s) according to further decoded information and filter parameters.

The entropy encoder 330, 430 receives the output of the prediction error encoder 303, 403 and may perform a suitable entropy encoding/variable length encoding on the signal to provide error detection and correction capability. The outputs of the entropy encoders 330, 430 may be inserted into a bitstream e.g. by a multiplexer 508.

Reference frames used in encoding may be stored to the reference frame memory 318. Each reference frame may be included in one or more of the reference picture lists, within a reference picture list, each entry has a reference index which identifies the reference frame. When a reference frame is no longer used as a reference frame it may be removed from the reference frame memory or marked as “unused for reference” or a non-reference frame wherein the storage location of that reference frame may be occupied for a new reference frame.

The H.264/AVC standard was developed by the Joint Video Team (JVT) of the Video Coding Experts Group (VCEG) of the Telecommunications Standardization Sector of International Telecommunication Union (ITU-T) and the Moving Picture Experts Group (MPEG) of International Organisation for Standardization (ISO)/International Electrotechnical Commission (IEC). The H.264/AVC standard is published by both parent standardization organizations, and it is referred to as ITU-T Recommendation H.264 and ISO/IEC International Standard 14496-10, also known as MPEG-4 Part 10 Advanced Video Coding (AVC). There have been multiple versions of the H.264/AVC standard, each integrating new extensions or features to the specification. These extensions include Scalable Video Coding (SVC) and Multiview Video Coding (MVC).

The High Efficiency Video Coding standard (which may be referred to as HEVC or H.265/HEVC) was developed by the Joint Collaborative Team-Video Coding (JCT-VC) of VCEG and MPEG. The standard is referred to as ITU-T Recommendation H.265 and ISO/IEC International Standard 23008-2, also known as MPEG-H Part 2 High Efficiency Video Coding (HEVC). There are currently ongoing standardization projects to develop extensions to H.265/HEVC, including scalable, multiview, three-dimensional, and fidelity range extensions.

Some key definitions, bitstream and coding structures, and concepts of H.264/AVC and HEVC are described in this section as an example of a video encoder, decoder, encoding method, decoding method, and a bitstream structure, wherein the embodiments may be implemented. Some of the key definitions, bitstream and coding structures, and concepts of H.264/AVC are the same as in a HEVC standard—hence, they are described below jointly. The aspects of the invention are not limited to H.264/AVC or HEVC, but rather the description is given for one possible basis on top of which the invention may be partly or fully realized.

When describing H.264/AVC and HEVC as well as in example embodiments, common notation for arithmetic operators, logical operators, relational operators, bit-wise operators, assignment operators, and range notation e.g. as specified in H.264/AVC or a draft HEVC may be used. Furthermore, common mathematical functions e.g. as specified in H.264/AVC or a draft HEVC may be used and a common order of precedence and execution order (from left to right or from right to left) of operators e.g. as specified in H.264/AVC or a draft HEVC may be used.

When describing H.264/AVC and HEVC as well as in example embodiments, the following descriptors may be used to specify the parsing process of each syntax element.

-   -   b(8): byte having any pattern of bit string (8 bits).     -   se(v): signed integer Exp-Golomb-coded syntax element with the         left bit first.     -   u(n): unsigned integer using n bits. When n is “v” in the syntax         table, the number of bits varies in a manner dependent on the         value of other syntax elements. The parsing process for this         descriptor is specified by n next bits from the bitstream         interpreted as a binary representation of an unsigned integer         with the most significant bit written first.     -   ue(v): unsigned integer Exp-Golomb-coded syntax element with the         left bit first.

An Exp-Golomb bit string may be converted to a code number (codeNum) for example using the following table:

Bit string codeNum 1 0 0 1 0 1 0 1 1 2 0 0 1 0 0 3 0 0 1 0 1 4 0 0 1 1 0 5 0 0 1 1 1 6 0 0 0 1 0 0 0 7 0 0 0 1 0 0 1 8 0 0 0 1 0 1 0 9 . . . . . .

A code number corresponding to an Exp-Golomb bit string may be converted to se(v) for example using the following table:

codeNum syntax element value 0 0 1 1 2 −1 3 2 4 −2 5 3 6 −3 . . . . . .

When describing H.264/AVC and HEVC as well as in example embodiments, syntax structures, semantics of syntax elements, and decoding process may be specified as follows. Syntax elements in the bitstream are represented in bold type. Each syntax element is described by its name (all lower case letters with underscore characters), optionally its one or two syntax categories, and one or two descriptors for its method of coded representation. The decoding process behaves according to the value of the syntax element and to the values of previously decoded syntax elements. When a value of a syntax element is used in the syntax tables or the text, it appears in regular (i.e., not bold) type. In some cases the syntax tables may use the values of other variables derived from syntax elements values. Such variables appear in the syntax tables, or text, named by a mixture of lower case and upper case letter and without any underscore characters. Variables starting with an upper case letter are derived for the decoding of the current syntax structure and all depending syntax structures. Variables starting with an upper case letter may be used in the decoding process for later syntax structures without mentioning the originating syntax structure of the variable. Variables starting with a lower case letter are only used within the context in which they are derived. In some cases, “mnemonic” names for syntax element values or variable values are used interchangeably with their numerical values. Sometimes “mnemonic” names are used without any associated numerical values. The association of values and names is specified in the text. The names are constructed from one or more groups of letters separated by an underscore character. Each group starts with an upper case letter and may contain more upper case letters.

When describing H.264/AVC and HEVC as well as in example embodiments, a syntax structure may be specified using the following. A group of statements enclosed in curly brackets is a compound statement and is treated functionally as a single statement. A “while” structure specifies a test of whether a condition is true, and if true, specifies evaluation of a statement (or compound statement) repeatedly until the condition is no longer true. A “do . . . while” structure specifies evaluation of a statement once, followed by a test of whether a condition is true, and if true, specifies repeated evaluation of the statement until the condition is no longer true. An “if . . . else” structure specifies a test of whether a condition is true, and if the condition is true, specifies evaluation of a primary statement, otherwise, specifies evaluation of an alternative statement. The “else” part of the structure and the associated alternative statement is omitted if no alternative statement evaluation is needed. A “for” structure specifies evaluation of an initial statement, followed by a test of a condition, and if the condition is true, specifies repeated evaluation of a primary statement followed by a subsequent statement until the condition is no longer true.

Similarly to many earlier video coding standards, the bitstream syntax and semantics as well as the decoding process for error-free bitstreams are specified in H.264/AVC and HEVC. The encoding process is not specified, but encoders must generate conforming bitstreams. Bitstream and decoder conformance can be verified with the Hypothetical Reference Decoder (HRD). The standards contain coding tools that help in coping with transmission errors and losses, but the use of the tools in encoding is optional and no decoding process has been specified for erroneous bitstreams.

In the description of existing standards as well as in the description of example embodiments, a syntax element may be defined as an element of data represented in the bitstream. A syntax structure may be defined as zero or more syntax elements present together in the bitstream in a specified order.

A profile may be defined as a subset of the entire bitstream syntax that is specified by a decoding/coding standard or specification. Within the bounds imposed by the syntax of a given profile it is still possible to require a very large variation in the performance of encoders and decoders depending upon the values taken by syntax elements in the bitstream such as the specified size of the decoded pictures. In many applications, it might be neither practical nor economic to implement a decoder capable of dealing with all hypothetical uses of the syntax within a particular profile. In order to deal with this issue, levels may be used. A level may be defined as a specified set of constraints imposed on values of the syntax elements in the bitstream and variables specified in a decoding/coding standard or specification. These constraints may be simple limits on values. Alternatively or in addition, they may take the form of constraints on arithmetic combinations of values (e.g., picture width multiplied by picture height multiplied by number of pictures decoded per second). Other means for specifying constraints for levels may also be used. Some of the constraints specified in a level may for example relate to the maximum picture size, maximum bitrate and maximum data rate in terms of coding units, such as macroblocks, per a time period, such as a second. The same set of levels may be defined for all profiles. It may be preferable for example to increase interoperability of terminals implementing different profiles that most or all aspects of the definition of each level may be common across different profiles.

An elementary unit for the input to an H.264/AVC or HEVC encoder and the output of an H.264/AVC or HEVC decoder, respectively, is a picture. In H.264/AVC and HEVC, a picture may either be a frame or a field. A frame comprises a matrix of luma samples and possibly the corresponding chroma samples. A field is a set of alternate sample rows of a frame and may be used as encoder input, when the source signal is interlaced. Chroma pictures may be absent (and hence monochrome sampling may be in use) or may be subsampled when compared to luma pictures. Some chroma formats may be summarized as follows:

In monochrome sampling there is only one sample array, which may be nominally considered the luma array.

In 4:2:0 sampling, each of the two chroma arrays has half the height and half the width of the luma array.

In 4:2:2 sampling, each of the two chroma arrays has the same height and half the width of the luma array.

In 4:4:4 sampling when no separate color planes are in use, each of the two chroma arrays has the same height and width as the luma array.

In H.264/AVC and HEVC, it is possible to code sample arrays as separate color planes into the bitstream and respectively decode separately coded color planes from the bitstream. When separate color planes are in use, each one of them is separately processed (by the encoder and/or the decoder) as a picture with monochrome sampling.

When chroma subsampling is in use (e.g. 4:2:0 or 4:2:2 chroma sampling), the location of chroma samples with respect to luma samples may be determined in the encoder side (e.g. as pre-processing step or as part of encoding). The chroma sample positions with respect to luma sample positions may be pre-defined for example in a coding standard, such as H.264/AVC or HEVC, or may be indicated in the bitstream for example as part of VUI of H.264/AVC or HEVC.

A partitioning may be defined as a division of a set into subsets such that each element of the set is in exactly one of the subsets. A picture partitioning may be defined as a division of a picture into smaller non-overlapping units. A block partitioning may be defined as a division of a block into smaller non-overlapping units, such as sub-blocks. In some cases term block partitioning may be considered to cover multiple levels of partitioning, for example partitioning of a picture into slices, and partitioning of each slice into smaller units, such as macroblocks of H.264/AVC. It is noted that the same unit, such as a picture, may have more than one partitioning. For example, a coding unit of a draft HEVC standard may be partitioned into prediction units and separately by another quadtree into transform units.

In H.264/AVC, a macroblock is a 16×16 block of luma samples and the corresponding blocks of chroma samples. For example, in the 4:2:0 sampling pattern, a macroblock contains one 8×8 block of chroma samples per each chroma component. In H.264/AVC, a picture is partitioned to one or more slice groups, and a slice group contains one or more slices. In H.264/AVC, a slice consists of an integer number of macroblocks ordered consecutively in the raster scan within a particular slice group.

During the course of HEVC standardization the terminology for example on picture partitioning units has evolved. In the next paragraphs, some non-limiting examples of HEVC terminology are provided.

In some video codecs, such as High Efficiency Video Coding (HEVC) codec, video pictures are divided into coding units (CU) covering the area of the picture. A CU consists of one or more prediction units (PU) defining the prediction process for the samples within the CU and one or more transform units (TU) defining the prediction error coding process for the samples in the said CU. Typically, a CU consists of a square block of samples with a size selectable from a predefined set of possible CU sizes. A CU with the maximum allowed size may be named as LCU (largest coding unit) and the video picture is divided into non-overlapping LCUs. An LCU can be further split into a combination of smaller CUs, e.g. by recursively splitting the LCU and resultant CUs. Each resulting CU typically has at least one PU and at least one TU associated with it. Each PU and TU can be further split into smaller PUs and TUs in order to increase granularity of the prediction and prediction error coding processes, respectively. Each PU has prediction information associated with it defining what kind of a prediction is to be applied for the pixels within that PU (e.g. motion vector information for inter predicted PUs and intra prediction directionality information for intra predicted PUs). Similarly each TU is associated with information describing the prediction error decoding process for the samples within the said TU (including e.g. DCT coefficient information). It is typically signalled at CU level whether prediction error coding is applied or not for each CU. In the case there is no prediction error residual associated with the CU, it can be considered there are no TUs for the said CU. The division of the image into CUs, and division of CUs into PUs and TUs is typically signalled in the bitstream allowing the decoder to reproduce the intended structure of these units.

In the HEVC standard, a picture can be partitioned in tiles, which are rectangular and contain an integer number of LCUs. In the HEVC standard, the partitioning to tiles forms a regular grid, where heights and widths of tiles differ from each other by one LCU at the maximum. Tiles are ordered in the bitstream consecutively in the raster scan within the picture. A tile may contain an integer number of slices.

In the HEVC, a slice consists of an integer number of LCUs. The LCUs are scanned in the raster scan order of LCUs within tiles or within a picture, if tiles are not in use. A slice may contain an integer number of tiles or a slice can be contained in a tile. Within an LCU, the CUs have a specific scan order.

FIG. 7 shows an example of a picture consisting of two tiles partitioned into square coding units (solid lines) which have further been partitioned into rectangular prediction units (dashed lines).

The decoder reconstructs the output video by applying prediction means similar to the encoder to form a predicted representation of the pixel blocks (using the motion or spatial information created by the encoder and stored in the compressed representation) and prediction error decoding (inverse operation of the prediction error coding recovering the quantized prediction error signal in spatial pixel domain). After applying prediction and prediction error decoding means the decoder sums up the prediction and prediction error signals (pixel values) to form the output video frame. The decoder (and encoder) can also apply additional filtering means to improve the quality of the output video before passing it for display and/or storing it as prediction reference for the forthcoming frames in the video sequence.

In typical video codecs the motion information is indicated with motion vectors associated with each motion compensated image block. Each of these motion vectors represents the displacement of the image block in the picture to be coded (in the encoder side) or decoded (in the decoder side) and the prediction source block in one of the previously coded or decoded pictures. In order to represent motion vectors efficiently those are typically coded differentially with respect to block specific predicted motion vectors. In typical video codecs the predicted motion vectors are created in a predefined way, for example calculating the median of the encoded or decoded motion vectors of the adjacent blocks. Another way to create motion vector predictions is to generate a list of candidate predictions from adjacent blocks and/or co-located blocks in temporal reference pictures and signalling the chosen candidate as the motion vector predictor. In addition to predicting the motion vector values, the reference index of previously coded/decoded picture can be predicted. The reference index is typically predicted from adjacent blocks and/or or co-located blocks in temporal reference picture. Moreover, typical high efficiency video codecs employ an additional motion information coding/decoding mechanism, often called merging/merge mode, where all the motion field information, which includes motion vector and corresponding reference picture index for each available reference picture list, is predicted and used without any modification/correction. Similarly, predicting the motion field information is carried out using the motion field information of adjacent blocks and/or co-located blocks in temporal reference pictures and the used motion field information is signalled among a list of motion field candidate list filled with motion field information of available adjacent/co-located blocks.

In typical video codecs the prediction residual after motion compensation is first transformed with a transform kernel (like DCT) and then coded. The reason for this is that often there still exists some correlation among the residual and transform can in many cases help reduce this correlation and provide more efficient coding.

Typical video encoders utilize Lagrangian cost functions to find optimal coding modes, e.g. the desired Macroblock mode and associated motion vectors. This kind of cost function uses a weighting factor λ to tie together the (exact or estimated) image distortion due to lossy coding methods and the (exact or estimated) amount of information that is required to represent the pixel values in an image area: C=D+λR,  (1)

where C is the Lagrangian cost to be minimized, D is the image distortion (e.g. Mean Squared Error) with the mode and motion vectors considered, and R the number of bits needed to represent the required data to reconstruct the image block in the decoder (including the amount of data to represent the candidate motion vectors).

Video coding standards and specifications may allow encoders to divide a coded picture to coded slices or alike. In-picture prediction is typically disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture to independently decodable pieces. In H.264/AVC and HEVC, in-picture prediction may be disabled across slice boundaries. Thus, slices can be regarded as a way to split a coded picture into independently decodable pieces, and slices are therefore often regarded as elementary units for transmission. In many cases, encoders may indicate in the bitstream which types of in-picture prediction are turned off across slice boundaries, and the decoder operation takes this information into account for example when concluding which prediction sources are available. For example, samples from a neighboring macroblock or CU may be regarded as unavailable for intra prediction, if the neighboring macroblock or CU resides in a different slice.

In the following, slice types available in some coding standards are categorized.

A raster-scan-order-slice is a coded segment that consists of consecutive macroblocks or alike in raster scan order. For example, video packets of MPEG-4 Part 2 and groups of macroblocks (GOBs) starting with a non-empty GOB header in H.263 are examples of raster-scan-order slices.

A rectangular slice is a coded segment that consists of a rectangular area of macroblocks or alike. A rectangular slice may be higher than one macroblock or alike row and narrower than the entire picture width. H.263 includes an optional rectangular slice submode, and H.261 GOBs can also be considered as rectangular slices.

A flexible slice can contain any pre-defined macroblock (or alike) locations. The H.264/AVC codec allows grouping of macroblocks to more than one slice groups. A slice group can contain any macroblock locations, including non-adjacent macroblock locations. A slice in some profiles of H.264/AVC consists of at least one macroblock within a particular slice group in raster scan order.

The elementary unit for the output of an H.264/AVC or HEVC encoder and the input of an H.264/AVC or HEVC decoder, respectively, is a Network Abstraction Layer (NAL) unit. For transport over packet-oriented networks or storage into structured files, NAL units may be encapsulated into packets or similar structures. A bytestream format has been specified in H.264/AVC and HEVC for transmission or storage environments that do not provide framing structures. The bytestream format separates NAL units from each other by attaching a start code in front of each NAL unit. To avoid false detection of NAL unit boundaries, encoders run a byte-oriented start code emulation prevention algorithm, which adds an emulation prevention byte to the NAL unit payload if a start code would have occurred otherwise. In order to enable straightforward gateway operation between packet- and stream-oriented systems, start code emulation prevention may always be performed regardless of whether the bytestream format is in use or not. A NAL unit may be defined as a syntax structure containing an indication of the type of data to follow and bytes containing that data in the form of an RBSP interspersed as necessary with emulation prevention bytes. A raw byte sequence payload (RBSP) may be defined as a syntax structure containing an integer number of bytes that is encapsulated in a NAL unit. An RBSP is either empty or has the form of a string of data bits containing syntax elements followed by an RBSP stop bit and followed by zero or more subsequent bits equal to 0.

NAL units consist of a header and payload. In H.264/AVC and HEVC, the NAL unit header indicates the type of the NAL unit and whether a coded slice contained in the NAL unit is a part of a reference picture or a non-reference picture.

H.264/AVC NAL unit header includes a 2-bit nal ref idc syntax element, which when equal to 0 indicates that a coded slice contained in the NAL unit is a part of a non-reference picture and when greater than 0 indicates that a coded slice contained in the NAL unit is a part of a reference picture. The header for SVC and MVC NAL units may additionally contain various indications related to the scalability and multiview hierarchy.

In HEVC, a two-byte NAL unit header is used for all specified NAL unit types. The NAL unit header contains one reserved bit, a six-bit NAL unit type indication, a six-bit reserved field (called nuh_layer_id) and a three-bit temporal_id_plus1 indication for temporal level. The temporal_id_plus1 syntax element may be regarded as a temporal identifier for the NAL unit, and a zero-based TemporalId variable may be derived as follows: TemporalId=temporal_id_plus1−1. TemporalId equal to 0 corresponds to the lowest temporal level. The value of temporal_id_plus1 is required to be non-zero in order to avoid start code emulation involving the two NAL unit header bytes. The bitstream created by excluding all VCL NAL units having a TemporalId greater than or equal to a selected value and including all other VCL NAL units remains conforming. Consequently, a picture having TemporalId equal to TID does not use any picture having a TemporalId greater than TID as inter prediction reference. A sub-layer or a temporal sub-layer may be defined to be a temporal scalable layer of a temporal scalable bitstream, consisting of VCL NAL units with a particular value of the TemporalId variable and the associated non-VCL NAL units. Without loss of generality, in some example embodiments a variable LayerId is derived from the value of nuh_layer_id for example as follows: LayerId=nuh_layer_id. In the following, LayerId, nuh_layer_id and layer_id are used interchangeably unless otherwise indicated.

It is expected that nuh_layer_id and/or similar syntax elements in NAL unit header would carry information on the scalability hierarchy. For example, the LayerId value nuh_layer_id and/or similar syntax elements may be mapped to values of variables or syntax elements describing different scalability dimensions, such as quality_id or similar, dependency_id or similar, any other type of layer identifier, view order index or similar, view identifier, an indication whether the NAL unit concerns depth or texture i.e. depth_flag or similar, or an identifier similar to priority_id of SVC indicating a valid sub-bitstream extraction if all NAL units greater than a specific identifier value are removed from the bitstream. nuh_layer_id and/or similar syntax elements may be partitioned into one or more syntax elements indicating scalability properties. For example, a certain number of bits among nuh_layer_id and/or similar syntax elements may be used for dependency_id or similar, while another certain number of bits among nuh_layer_id and/or similar syntax elements may be used for quality_id or similar. Alternatively, a mapping of LayerId values or similar to values of variables or syntax elements describing different scalability dimensions may be provided for example in a Video Parameter Set, a Sequence Parameter Set or another syntax structure.

NAL units can be categorized into Video Coding Layer (VCL) NAL units and non-VCL NAL units. VCL NAL units are typically coded slice NAL units. In H.264/AVC, coded slice NAL units contain syntax elements representing one or more coded macroblocks, each of which corresponds to a block of samples in the uncompressed picture. In HEVC, coded slice NAL units contain syntax elements representing one or more CU.

In H.264/AVC and HEVC a coded slice NAL unit can be indicated to be a coded slice in an Instantaneous Decoding Refresh (IDR) picture or coded slice in a non-IDR picture

In HEVC, a coded slice NAL unit can be indicated to be one of the following types.

Content of NAL Name of unit and RBSP syntax nal_unit_type nal_unit_type structure  0, 1 TRAIL_N, Coded slice TRAIL_R segment of a non-TSA, non- STSA trailing picture slice_segment_layer_rbsp( )  2, 3 TSA_N, Coded slice TSA_R segment of a TSA picture slice_segment_layer_rbsp( )  4, 5 STSA_N, Coded slice STSA_R segment of an STSA picture slice_layer_rbsp( )  6, 7 RADL_N, Coded slice RADL_R segment of a RADL picture slice_layer_rbsp( )  8, 9 RASL_N, Coded slice RASL_R, segment of a RASL picture slice_layer_rbsp( ) 10, RSV_VCL_N10 Reserved // 12, RSV_VCL_N12 reserved non-RAP non- 14 RSV_VCL_N14 reference VCL NAL unit types 11, RSV_VCL_R11 Reserved // 13, RSV_VCL_R13 reserved non-RAP reference 15 RSV_VCL_R15 VCL NAL unit types 16, BLA_W_LP Coded slice 17, BLA_W_DLP segment of a BLA picture 18 BLA_N_LP slice_segment_layer_rbsp( ) 19, IDR_W_DLP Coded slice 20 IDR_N_LP segment of an IDR picture slice_segment_layer_rbsp( ) 21 CRA_NUT Coded slice segment of a CRA picture slice_segment_layer_rbsp( ) 22, RSV_RAP_VCL22.. Reserved // 23 RSV_RAP_VCL23 reserved RAP VCL NAL unit types 24..31 RSV_VCL24.. Reserved // RSV_VCL31 reserved non-RAP VCL NAL unit types

In HEVC, abbreviations for picture types may be defined as follows: trailing (TRAIL) picture, Temporal Sub-layer Access (TSA), Step-wise Temporal Sub-layer Access (STSA), Random Access Decodable Leading (RADL) picture, Random Access Skipped Leading (RASL) picture, Broken Link Access (BLA) picture, Instantaneous Decoding Refresh (IDR) picture, Clean Random Access (CRA) picture.

A Random Access Point (RAP) picture, which may also or alternatively be referred to as intra random access point (IRAP) picture, is a picture where each slice or slice segment has nal_unit_type in the range of 16 to 23, inclusive. A RAP picture contains only intra-coded slices, and may be a BLA picture, a CRA picture or an IDR picture. The first picture in the bitstream is a RAP picture. Provided the necessary parameter sets are available when they need to be activated, the RAP picture and all subsequent non-RASL pictures in decoding order can be correctly decoded without performing the decoding process of any pictures that precede the RAP picture in decoding order. There may be pictures in a bitstream that contain only intra-coded slices that are not RAP pictures.

In HEVC a CRA picture may be the first picture in the bitstream in decoding order, or may appear later in the bitstream. CRA pictures in HEVC allow so-called leading pictures that follow the CRA picture in decoding order but precede it in output order. Some of the leading pictures, so-called RASL pictures, may use pictures decoded before the CRA picture as a reference. Pictures that follow a CRA picture in both decoding and output order are decodable if random access is performed at the CRA picture, and hence clean random access is achieved similarly to the clean random access functionality of an IDR picture.

A CRA picture may have associated RADL or RASL pictures. When a CRA picture is the first picture in the bitstream in decoding order, the CRA picture is the first picture of a coded video sequence in decoding order, and any associated RASL pictures are not output by the decoder and may not be decodable, as they may contain references to pictures that are not present in the bitstream.

A leading picture is a picture that precedes the associated RAP picture in output order. The associated RAP picture is the previous RAP picture in decoding order (if present). A leading picture may either be a RADL picture or a RASL picture.

All RASL pictures are leading pictures of an associated BLA or CRA picture. When the associated RAP picture is a BLA picture or is the first coded picture in the bitstream, the RASL picture is not output and may not be correctly decodable, as the RASL picture may contain references to pictures that are not present in the bitstream. However, a RASL picture can be correctly decoded if the decoding had started from a RAP picture before the associated RAP picture of the RASL picture. RASL pictures are not used as reference pictures for the decoding process of non-RASL pictures. When present, all RASL pictures precede, in decoding order, all trailing pictures of the same associated RAP picture. In some earlier drafts of the HEVC standard, a RASL picture was referred to a Tagged for Discard (TFD) picture.

All RADL pictures are leading pictures. RADL pictures are not used as reference pictures for the decoding process of trailing pictures of the same associated RAP picture. When present, all RADL pictures precede, in decoding order, all trailing pictures of the same associated RAP picture. RADL pictures do not refer to any picture preceding the associated RAP picture in decoding order and can therefore be correctly decoded when the decoding starts from the associated RAP picture. In some earlier drafts of the HEVC standard, a RADL picture was referred to a Decodable Leading Picture (DLP).

Decodable leading pictures may be such that can be correctly decoded when the decoding is started from the CRA picture. In other words, decodable leading pictures use only the initial CRA picture or subsequent pictures in decoding order as reference in inter prediction. Non-decodable leading pictures are such that cannot be correctly decoded when the decoding is started from the initial CRA picture. In other words, non-decodable leading pictures use pictures prior, in decoding order, to the initial CRA picture as references in inter prediction.

When a part of a bitstream starting from a CRA picture is included in another bitstream, the RASL pictures associated with the CRA picture might not be correctly decodable, because some of their reference pictures might not be present in the combined bitstream. To make such a splicing operation straightforward, the NAL unit type of the CRA picture can be changed to indicate that it is a BLA picture. The RASL pictures associated with a BLA picture may not be correctly decodable hence are not be output/displayed. Furthermore, the RASL pictures associated with a BLA picture may be omitted from decoding.

A BLA picture may be the first picture in the bitstream in decoding order, or may appear later in the bitstream. Each BLA picture begins a new coded video sequence, and has similar effect on the decoding process as an IDR picture. However, a BLA picture contains syntax elements that specify a non-empty reference picture set. When a BLA picture has nal_unit_type equal to BLA_W_LP, it may have associated RASL pictures, which are not output by the decoder and may not be decodable, as they may contain references to pictures that are not present in the bitstream. When a BLA picture has nal_unit_type equal to BLA_W_LP, it may also have associated RADL pictures, which are specified to be decoded. When a BLA picture has nal_unit_type equal to BLA_W_DLP, it does not have associated RASL pictures but may have associated RADL pictures, which are specified to be decoded. BLA_W_DLP may also be referred to as BLA_W_RADL. When a BLA picture has nal_unit_type equal to BLA_N_LP, it does not have any associated leading pictures.

An IDR picture having nal_unit_type equal to IDR_N_LP does not have associated leading pictures present in the bitstream. An IDR picture having nal_unit_type equal to IDR_W_DLP does not have associated RASL pictures present in the bitstream, but may have associated RADL pictures in the bitstream. IDR_W_DLP may also be referred to as IDR_W_RADL.

When the value of nal_unit_type is equal to TRAIL_N, TSA_N, STSA_N, RADL_N, RASL_N, RSV_VCL_N10, RSV_VCL_N12, or RSV_VCL_N14, the decoded picture is not used as a reference for any other picture of the same temporal sub-layer. That is, in a draft HEVC standard, when the value of nal_unit_type is equal to TRAIL_N, TSA_N, STSA_N, RADL_N, RASL_N, RSV_VCL_N10, RSV_VCL_N12, or RSV_VCL_N14, the decoded picture is not included in any of RefPicSetStCurrBefore, RefPicSetStCurrAfter and RefPicSetLtCurr of any picture with the same value of TemporalId. A coded picture with nal_unit_type equal to TRAIL_N, TSA_N, STSA_N, RADL_N, RASL_N, RSV_VCL_N10, RSV_VCL_N12, or RSV_VCL_N14 may be discarded without affecting the decodability of other pictures with the same value of TemporalId.

A trailing picture may be defined as a picture that follows the associated RAP picture in output order. Any picture that is a trailing picture does not have nal_unit_type equal to RADL_N, RADL_R, RASL_N or RASL_R. Any picture that is a leading picture may be constrained to precede, in decoding order, all trailing pictures that are associated with the same RAP picture. No RASL pictures are present in the bitstream that are associated with a BLA picture having nal_unit_type equal to BLA_W_DLP or BLA_N_LP. No RADL pictures are present in the bitstream that are associated with a BLA picture having nal_unit_type equal to BLA_N_LP or that are associated with an IDR picture having nal_unit_type equal to IDR_N_LP. Any RASL picture associated with a CRA or BLA picture may be constrained to precede any RADL picture associated with the CRA or BLA picture in output order. Any RASL picture associated with a CRA picture may be constrained to follow, in output order, any other RAP picture that precedes the CRA picture in decoding order.

In HEVC there are two picture types, the TSA and STSA picture types, that can be used to indicate temporal sub-layer switching points. If temporal sub-layers with TemporalId up to N had been decoded until the TSA or STSA picture (exclusive) and the TSA or STSA picture has TemporalId equal to N+1, the TSA or STSA picture enables decoding of all subsequent pictures (in decoding order) having TemporalId equal to N+1. The TSA picture type may impose restrictions on the TSA picture itself and all pictures in the same sub-layer that follow the TSA picture in decoding order. None of these pictures is allowed to use inter prediction from any picture in the same sub-layer that precedes the TSA picture in decoding order. The TSA definition may further impose restrictions on the pictures in higher sub-layers that follow the TSA picture in decoding order. None of these pictures is allowed to refer a picture that precedes the TSA picture in decoding order if that picture belongs to the same or higher sub-layer as the TSA picture. TSA pictures have TemporalId greater than 0. The STSA is similar to the TSA picture but does not impose restrictions on the pictures in higher sub-layers that follow the STSA picture in decoding order and hence enable up-switching only onto the sub-layer where the STSA picture resides.

A non-VCL NAL unit may be for example one of the following types: a sequence parameter set, a picture parameter set, a supplemental enhancement information (SEI) NAL unit, an access unit delimiter, an end of sequence NAL unit, an end of stream NAL unit, or a filler data NAL unit. Parameter sets may be needed for the reconstruction of decoded pictures, whereas many of the other non-VCL NAL units are not necessary for the reconstruction of decoded sample values.

Parameters that remain unchanged through a coded video sequence may be included in a sequence parameter set. In addition to the parameters that may be needed by the decoding process, the sequence parameter set may optionally contain video usability information (VUI), which includes parameters that may be important for buffering, picture output timing, rendering, and resource reservation. There are three NAL units specified in H.264/AVC to carry sequence parameter sets: the sequence parameter set NAL unit containing all the data for H.264/AVC VCL NAL units in the sequence, the sequence parameter set extension NAL unit containing the data for auxiliary coded pictures, and the subset sequence parameter set for MVC and SVC VCL NAL units. In the HEVC standard a sequence parameter set RBSP includes parameters that can be referred to by one or more picture parameter set RBSPs or one or more SEI NAL units containing a buffering period SEI message. A picture parameter set contains such parameters that are likely to be unchanged in several coded pictures. A picture parameter set RBSP may include parameters that can be referred to by the coded slice NAL units of one or more coded pictures.

An Adaptation Parameter Set (APS), which includes parameters that are likely to be unchanged in several coded slices but may change for example for each picture or each few pictures was proposed for HEVC but eventually not adopted into the standard. The APS syntax structure has been proposed to include parameters or syntax elements related to quantization matrices (QM), adaptive sample offset (SAO), adaptive loop filtering (ALF), and deblocking filtering. An APS may also be a NAL unit and coded without reference or prediction from any other NAL unit. An identifier, referred to as aps_id syntax element, is included in APS NAL unit, and included and used in the slice header to refer to a particular APS. An APS syntax structure may only contain ALF parameters.

The HEVC standard also includes a video parameter set (VPS) NAL unit. A video parameter set RBSP may include parameters that can be referred to by one or more sequence parameter set RBSPs.

The relationship and hierarchy between video parameter set (VPS), sequence parameter set (SPS), and picture parameter set (PPS) may be described as follows. VPS resides one level above SPS in the parameter set hierarchy and in the context of scalability and/or 3DV. VPS may include parameters that are common for all slices across all (scalability or view) layers in the entire coded video sequence. SPS includes the parameters that are common for all slices in a particular (scalability or view) layer in the entire coded video sequence, and may be shared by multiple (scalability or view) layers. PPS includes the parameters that are common for all slices in a particular layer representation (the representation of one scalability or view layer in one access unit) and are likely to be shared by all slices in multiple layer representations.

VPS may provide information about the dependency relationships of the layers in a bitstream, as well as many other information that are applicable to all slices across all (scalability or view) layers in the entire coded video sequence. In a scalable extension of HEVC, VPS may for example include a mapping of the LayerId value derived from the NAL unit header to one or more scalability dimension values, for example correspond to dependency_id, quality_id, view_id, and depth_flag for the layer defined similarly to SVC and MVC. VPS may include profile and level information for one or more layers as well as the profile and/or level for one or more temporal sub-layers (consisting of VCL NAL units at and below certain temporal_id values) of a layer representation.

H.264/AVC and HEVC syntax allows many instances of parameter sets, and each instance is identified with a unique identifier. In order to limit the memory usage needed for parameter sets, the value range for parameter set identifiers has been limited. In H.264/AVC and HEVC standard, each slice header includes the identifier of the picture parameter set that is active for the decoding of the picture that contains the slice, and each picture parameter set contains the identifier of the active sequence parameter set. Consequently, the transmission of picture and sequence parameter sets does not have to be accurately synchronized with the transmission of slices. Instead, it is sufficient that the active sequence and picture parameter sets are received at any moment before they are referenced, which allows transmission of parameter sets “out-of-band” using a more reliable transmission mechanism compared to the protocols used for the slice data. For example, parameter sets can be included as a parameter in the session description for Real-time Transport Protocol (RTP) sessions. If parameter sets are transmitted in-band, they can be repeated to improve error robustness.

A parameter set may be activated by a reference from a slice or from another active parameter set or in some cases from another syntax structure such as a buffering period SEI message.

A SEI NAL unit may contain one or more SEI messages, which are not required for the decoding of output pictures but may assist in related processes, such as picture output timing, rendering, error detection, error concealment, and resource reservation. Several SEI messages are specified in H.264/AVC and HEVC, and the user data SEI messages enable organizations and companies to specify SEI messages for their own use. H.264/AVC and HEVC contain the syntax and semantics for the specified SEI messages but no process for handling the messages in the recipient is defined. Consequently, encoders are required to follow the H.264/AVC standard or the HEVC standard when they create SEI messages, and decoders conforming to the H.264/AVC standard or the HEVC standard, respectively, are not required to process SEI messages for output order conformance. One of the reasons to include the syntax and semantics of SEI messages in H.264/AVC and HEVC is to allow different system specifications to interpret the supplemental information identically and hence interoperate. It is intended that system specifications can require the use of particular SEI messages both in the encoding end and in the decoding end, and additionally the process for handling particular SEI messages in the recipient can be specified.

A coded picture is a coded representation of a picture. A coded picture in H.264/AVC comprises the VCL NAL units that are required for the decoding of the picture. In H.264/AVC, a coded picture can be a primary coded picture or a redundant coded picture. A primary coded picture is used in the decoding process of valid bitstreams, whereas a redundant coded picture is a redundant representation that should only be decoded when the primary coded picture cannot be successfully decoded. In the HEVC, no redundant coded picture has been specified.

In H.264/AVC, an access unit comprises a primary coded picture and those NAL units that are associated with it. In HEVC, an access unit is defined as a set of NAL units that are associated with each other according to a specified classification rule, are consecutive in decoding order, and contain exactly one coded picture. In H.264/AVC, the appearance order of NAL units within an access unit is constrained as follows. An optional access unit delimiter NAL unit may indicate the start of an access unit. It is followed by zero or more SEI NAL units. The coded slices of the primary coded picture appear next. In H.264/AVC, the coded slice of the primary coded picture may be followed by coded slices for zero or more redundant coded pictures. A redundant coded picture is a coded representation of a picture or a part of a picture. A redundant coded picture may be decoded if the primary coded picture is not received by the decoder for example due to a loss in transmission or a corruption in physical storage medium.

In H.264/AVC, an access unit may also include an auxiliary coded picture, which is a picture that supplements the primary coded picture and may be used for example in the display process. An auxiliary coded picture may for example be used as an alpha channel or alpha plane specifying the transparency level of the samples in the decoded pictures. An alpha channel or plane may be used in a layered composition or rendering system, where the output picture is formed by overlaying pictures being at least partly transparent on top of each other. An auxiliary coded picture has the same syntactic and semantic restrictions as a monochrome redundant coded picture. In H.264/AVC, an auxiliary coded picture contains the same number of macroblocks as the primary coded picture.

In H.264/AVC, a coded video sequence is defined to be a sequence of consecutive access units in decoding order from an IDR access unit, inclusive, to the next IDR access unit, exclusive, or to the end of the bitstream, whichever appears earlier. In HEVC, a coded video sequence may be defined to be a sequence of access units that consists, in decoding order, of a CRA access unit that is the first access unit in the bitstream, an IDR access unit or a BLA access unit, followed by zero or more non-IDR and non-BLA access units including all subsequent access units up to but not including any subsequent IDR or BLA access unit.

A group of pictures (GOP) and its characteristics may be defined as follows. A GOP can be decoded regardless of whether any previous pictures were decoded. An open GOP is such a group of pictures in which pictures preceding the initial intra picture in output order might not be correctly decodable when the decoding starts from the initial intra picture of the open GOP. In other words, pictures of an open GOP may refer (in inter prediction) to pictures belonging to a previous GOP. An H.264/AVC decoder can recognize an intra picture starting an open GOP from the recovery point SEI message in an H.264/AVC bitstream. An HEVC decoder can recognize an intra picture starting an open GOP, because a specific NAL unit type, CRA NAL unit type, is used for its coded slices. A closed GOP is such a group of pictures in which all pictures can be correctly decoded when the decoding starts from the initial intra picture of the closed GOP. In other words, no picture in a closed GOP refers to any pictures in previous GOPs. In H.264/AVC and HEVC, a closed GOP starts from an IDR access unit. In HEVC a closed GOP may also start from a BLA_W_DLP or a BLA_N_LP picture. As a result, closed GOP structure has more error resilience potential in comparison to the open GOP structure, however at the cost of possible reduction in the compression efficiency. Open GOP coding structure is potentially more efficient in the compression, due to a larger flexibility in selection of reference pictures.

The bitstream syntax of H.264/AVC and HEVC indicates whether a particular picture is a reference picture for inter prediction of any other picture. Pictures of any coding type (I, P, B) can be reference pictures or non-reference pictures in H.264/AVC and HEVC.

H.264/AVC specifies the process for decoded reference picture marking in order to control the memory consumption in the decoder. The maximum number of reference pictures used for inter prediction, referred to as M, is determined in the sequence parameter set. When a reference picture is decoded, it is marked as “used for reference”. If the decoding of the reference picture caused more than M pictures marked as “used for reference”, at least one picture is marked as “unused for reference”. There are two types of operation for decoded reference picture marking: adaptive memory control and sliding window. The operation mode for decoded reference picture marking is selected on picture basis. The adaptive memory control enables explicit signaling which pictures are marked as “unused for reference” and may also assign long-term indices to short-term reference pictures. The adaptive memory control may require the presence of memory management control operation (MMCO) parameters in the bitstream. MMCO parameters may be included in a decoded reference picture marking syntax structure. If the sliding window operation mode is in use and there are M pictures marked as “used for reference”, the short-term reference picture that was the first decoded picture among those short-term reference pictures that are marked as “used for reference” is marked as “unused for reference”. In other words, the sliding window operation mode results into first-in-first-out buffering operation among short-term reference pictures.

One of the memory management control operations in H.264/AVC causes all reference pictures except for the current picture to be marked as “unused for reference”. An instantaneous decoding refresh (IDR) picture contains only intra-coded slices and causes a similar “reset” of reference pictures.

In the HEVC standard, reference picture marking syntax structures and related decoding processes are not used, but instead a reference picture set (RPS) syntax structure and decoding process are used instead for a similar purpose. A reference picture set valid or active for a picture includes all the reference pictures used as reference for the picture and all the reference pictures that are kept marked as “used for reference” for any subsequent pictures in decoding order. There are six subsets of the reference picture set, which are referred to as namely RefPicSetStCurr0, RefPicSetStCurr1, RefPicSetStFoll0, RefPicSetStFoll1, RefPicSetLtCurr, and RefPicSetLtFoll. The notation of the six subsets is as follows. “Curr” refers to reference pictures that are included in the reference picture lists of the current picture and hence may be used as inter prediction reference for the current picture. “Foll” refers to reference pictures that are not included in the reference picture lists of the current picture but may be used in subsequent pictures in decoding order as reference pictures. “St” refers to short-term reference pictures, which may generally be identified through a certain number of least significant bits of their POC value. “Lt” refers to long-term reference pictures, which are specifically identified and generally have a greater difference of POC values relative to the current picture than what can be represented by the mentioned certain number of least significant bits. “0” refers to those reference pictures that have a smaller POC value than that of the current picture. “1” refers to those reference pictures that have a greater POC value than that of the current picture. RefPicSetStCurr0, RefPicSetStCurr1, RefPicSetStFoll0 and RefPicSetStFoll1 are collectively referred to as the short-term subset of the reference picture set. RefPicSetLtCurr and RefPicSetLtFoll are collectively referred to as the long-term subset of the reference picture set.

In the HEVC standard, a reference picture set may be specified in a sequence parameter set and taken into use in the slice header through an index to the reference picture set. A reference picture set may also be specified in a slice header. A long-term subset of a reference picture set is generally specified only in a slice header, while the short-term subsets of the same reference picture set may be specified in the picture parameter set or slice header. A reference picture set may be coded independently or may be predicted from another reference picture set (known as inter-RPS prediction). When a reference picture set is independently coded, the syntax structure includes up to three loops iterating over different types of reference pictures; short-term reference pictures with lower POC value than the current picture, short-term reference pictures with higher POC value than the current picture and long-term reference pictures. Each loop entry specifies a picture to be marked as “used for reference”. In general, the picture is specified with a differential POC value. The inter-RPS prediction exploits the fact that the reference picture set of the current picture can be predicted from the reference picture set of a previously decoded picture. This is because all the reference pictures of the current picture are either reference pictures of the previous picture or the previously decoded picture itself. It is only necessary to indicate which of these pictures should be reference pictures and be used for the prediction of the current picture. In both types of reference picture set coding, a flag (used_by_curr_pic_X_flag) is additionally sent for each reference picture indicating whether the reference picture is used for reference by the current picture (included in a *Curr list) or not (included in a *Foll list). Pictures that are included in the reference picture set used by the current slice are marked as “used for reference”, and pictures that are not in the reference picture set used by the current slice are marked as “unused for reference”. If the current picture is an IDR picture, RefPicSetStCurr0, RefPicSetStCurr1, RefPicSetStFoll0, RefPicSetStFoll1, RefPicSetLtCurr, and RefPicSetLtFoll are all set to empty.

A Decoded Picture Buffer (DPB) may be used in the encoder and/or in the decoder. There are two reasons to buffer decoded pictures, for references in inter prediction and for reordering decoded pictures into output order. As H.264/AVC and HEVC provide a great deal of flexibility for both reference picture marking and output reordering, separate buffers for reference picture buffering and output picture buffering may waste memory resources. Hence, the DPB may include a unified decoded picture buffering process for reference pictures and output reordering. A decoded picture may be removed from the DPB when it is no longer used as a reference and is not needed for output.

In many coding modes of H.264/AVC and HEVC, the reference picture for inter prediction is indicated with an index to a reference picture list. The index may be coded with variable length coding, which usually causes a smaller index to have a shorter value for the corresponding syntax element. In H.264/AVC and HEVC, two reference picture lists (reference picture list 0 and reference picture list 1) are generated for each bi-predictive (B) slice, and one reference picture list (reference picture list 0) is formed for each inter-coded (P) slice.

A reference picture list, such as reference picture list 0 and reference picture list 1, is typically constructed in two steps: First, an initial reference picture list is generated. The initial reference picture list may be generated for example on the basis of frame_num, POC, temporal_id, or information on the prediction hierarchy such as GOP structure, or any combination thereof. Second, the initial reference picture list may be reordered by reference picture list reordering (RPLR) commands, also known as reference picture list modification syntax structure, which may be contained in slice headers. The RPLR commands indicate the pictures that are ordered to the beginning of the respective reference picture list. This second step may also be referred to as the reference picture list modification process, and the RPLR commands may be included in a reference picture list modification syntax structure. If reference picture sets are used, the reference picture list 0 may be initialized to contain RefPicSetStCurr0 first, followed by RefPicSetStCurr1, followed by RefPicSetLtCurr. Reference picture list 1 may be initialized to contain RefPicSetStCurr1 first, followed by RefPicSetStCurr0. The initial reference picture lists may be modified through the reference picture list modification syntax structure, where pictures in the initial reference picture lists may be identified through an entry index to the list.

A way of categorizing different types of prediction that may be applied in video encoding and/or video decoding is to consider whether prediction applies to sample values or (de)coding parameters.

In the sample prediction, pixel or sample values in a certain picture area or “block” are predicted. These pixel or sample values can be predicted, for example, using one or more of the following ways:

Motion compensation mechanisms (which may also be referred to as temporal prediction or motion-compensated temporal prediction or motion-compensated prediction or MCP), which involve finding and indicating an area in one of the previously encoded video frames that corresponds closely to the block being coded.

Inter-view prediction, which involves finding and indicating an area in one of the previously encoded view components that corresponds closely to the block being coded.

View synthesis prediction, which involves synthesizing a prediction block or image area where a prediction block is derived on the basis of reconstructed/decoded ranging information.

Inter-layer prediction using reconstructed/decoded samples, such as the so-called IntraBL (base layer) mode of SVC.

Inter-layer residual prediction, in which for example the coded residual of a reference layer or a derived residual from a difference of a reconstructed/decoded reference layer picture and a corresponding reconstructed/decoded enhancement layer picture may be used for predicting a residual block of the current enhancement layer block. A residual block may be added for example to a motion-compensated prediction block to obtain a final prediction block for the current enhancement layer block. Residual prediction may sometimes be treated as a separate type of prediction in addition to sample and syntax prediction.

Intra prediction, where pixel or sample values can be predicted by spatial mechanisms which involve finding and indicating a spatial region relationship.

In the syntax prediction, which may also be referred to as parameter prediction, syntax elements and/or syntax element values and/or variables derived from syntax elements are predicted from syntax elements (de)coded earlier and/or variables derived earlier. Non-limiting examples of syntax prediction are provided below:

In motion vector prediction, motion vectors e.g. for inter and/or inter-view prediction may be coded differentially with respect to a block-specific predicted motion vector. In many video codecs, the predicted motion vectors are created in a predefined way, for example by calculating the median of the encoded or decoded motion vectors of the adjacent blocks. Another way to create motion vector predictions, sometimes referred to as advanced motion vector prediction (AMVP), is to generate a list of candidate predictions from adjacent blocks and/or co-located blocks in temporal reference pictures and signalling the chosen candidate as the motion vector predictor. In addition to predicting the motion vector values, the reference index of a previously coded/decoded picture can be predicted. The reference index may be predicted from adjacent blocks and/or co-located blocks in a temporal reference picture. Differential coding of motion vectors may be disabled across slice boundaries.

The block partitioning, e.g. from CTU to CUs and down to PUs, may be predicted.

In filter parameter prediction, the filtering parameters e.g. for sample adaptive offset may be predicted.

Another way of categorizing different types of prediction that may be applied in video encoding and/or video decoding is to consider across which domains or scalability types the prediction crosses. This categorization may lead into one or more of the following types of prediction, which may also sometimes be referred to as prediction directions:

Temporal prediction e.g. of sample values or motion vectors from an earlier picture usually of the same scalability layer, view and component type (texture or depth).

Inter-view prediction (which may also be referred to as cross-view prediction) referring to prediction taking place between view components usually of the same time instant or access unit and the same component type.

Inter-layer prediction referring to prediction taking place between layers usually of the same time instant, of the same component type, and of the same view.

Inter-component prediction may be defined to comprise prediction of syntax element values, sample values, variable values used in the decoding process, or anything alike from a component picture of one type to a component picture of another type. For example, inter-component prediction may comprise prediction of a texture view component from a depth view component, or vice versa.

Prediction approaches using image information from a previously coded image can also be called as inter prediction methods. Inter prediction may sometimes be considered to only include motion-compensated temporal prediction, while it may sometimes be considered to include all types of prediction where a reconstructed/decoded block of samples is used as a prediction source, therefore including conventional inter-view prediction for example. Inter prediction may be considered to comprise only sample prediction but it may alternatively be considered to comprise both sample and syntax prediction. As a result of syntax and sample prediction, a predicted block of pixels of samples may be obtained.

Prediction approaches using image information within the same image can also be called as intra prediction methods. Intra prediction may be considered to comprise only sample prediction but it may alternatively be considered to comprise both sample and syntax prediction.

If the prediction, such as predicted variable values and/or prediction blocks, is not refined by the encoder using any form of prediction error or residual coding, prediction may be referred to as inheritance.

Multi-view coding has been realized as a multi-loop scalable video coding scheme, where the inter-view reference pictures are added into the reference picture lists. In MVC the inter-view reference components and inter-view only reference components that are included in the reference picture lists may be considered as not being marked as “used for short-term reference” or “used for long-term reference”. In the derivation of temporal direct luma motion vector, the co-located motion vector may not be scaled if the picture order count difference of List 1 reference (from which the co-located motion vector is obtained) and List 0 reference is 0, i.e. if td is equal to 0 in FIG. 6 c.

FIG. 6a illustrates an example of spatial and temporal prediction of a prediction unit. There is depicted the current block 601 in the frame 600 and a neighbor block 602 which already has been encoded. A motion vector definer 362 (FIG. 4a ) has defined a motion vector 603 for the neighbor block 602 which points to a block 604 in the previous frame 605. This motion vector can be used as a potential spatial motion vector prediction 610 for the current block. FIG. 6a depicts that a co-located block 606 in the previous frame 605, i.e. the block at the same location than the current block but in the previous frame, has a motion vector 607 pointing to a block 609 in another frame 608. This motion vector 607 can be used as a potential temporal motion vector prediction 611 for the current block 601.

FIG. 6b illustrates another example of spatial and temporal prediction of a prediction unit. In this example the block 606 of the previous frame 605 uses bi-directional prediction based on the block 609 of the frame 608 preceding the frame 605 and on the block 612 in the frame 613 succeeding the current frame 600. The temporal motion vector prediction for the current block 601 may be formed by using both motion vectors 607, 614 or either of them.

In HEVC temporal motion vector prediction (TMVP), the reference picture list to be used for obtaining a collocated partition is chosen according to the collocated_from_10_flag syntax element in the slice header. When the flag is equal to 1, it specifies that the picture that contains the collocated partition is derived from list 0, otherwise the picture is derived from list 1. When collocated_from_10_flag is not present, it is inferred to be equal to 1. The collocated_ref_idx in the slice header specifies the reference index of the picture that contains the collocated partition. When the current slice is a P slice, collocated_ref_idx refers to a picture in list 0. When the current slice is a B slice, collocated_ref_idx refers to a picture in list 0 if collocated_from_10 is 1, otherwise it refers to a picture in list 1. collocated_ref_idx always refers to a valid list entry, and the resulting picture is the same for all slices of a coded picture. When collocated_ref_idx is not present, it is inferred to be equal to 0.

In HEVC, when the current PU uses the merge mode, the target reference index for TMVP is set to 0 (for both reference picture list 0 and 1). In AMVP, the target reference index is indicated in the bitstream.

In HEVC, the availability of a candidate predicted motion vector (PMV) for the merge mode may be determined as follows (both for spatial and temporal candidates) (SRTP=short-term reference picture, LRTP=long-term reference picture)

reference picture for target reference picture for candidate PMV reference index candidate PMV availability STRP STRP “available” (and scaled) STRP LTRP “unavailable” LTRP STRP “unavailable” LTRP LTRP “available” but not scaled

Motion vector scaling may be performed in the case both target reference picture and the reference index for candidate PMV are short-term reference pictures. The scaling may be performed by scaling the motion vector with appropriate POC differences related to the candidate motion vector and the target reference picture relative to the current picture, e.g. with the POC difference of the current picture and the target reference picture divided by the POC difference of the current picture and the POC difference of the picture containing the candidate PMV and its reference picture.

A coding technique known as isolated regions is based on constraining in-picture prediction and inter prediction jointly. An isolated region in a picture can contain any macroblock (or alike) locations, and a picture can contain zero or more isolated regions that do not overlap. A leftover region, if any, is the area of the picture that is not covered by any isolated region of a picture. When coding an isolated region, at least some types of in-picture prediction is disabled across its boundaries. A leftover region may be predicted from isolated regions of the same picture.

A coded isolated region can be decoded without the presence of any other isolated or leftover region of the same coded picture. It may be necessary to decode all isolated regions of a picture before the leftover region. In some implementations, an isolated region or a leftover region contains at least one slice.

Pictures, whose isolated regions are predicted from each other, may be grouped into an isolated-region picture group. An isolated region can be inter-predicted from the corresponding isolated region in other pictures within the same isolated-region picture group, whereas inter prediction from other isolated regions or outside the isolated-region picture group may be disallowed. A leftover region may be inter-predicted from any isolated region. The shape, location, and size of coupled isolated regions may evolve from picture to picture in an isolated-region picture group.

Coding of isolated regions in the H.264/AVC codec may be based on slice groups. The mapping of macroblock locations to slice groups may be specified in the picture parameter set. The H.264/AVC syntax includes syntax to code certain slice group patterns, which can be categorized into two types, static and evolving. The static slice groups stay unchanged as long as the picture parameter set is valid, whereas the evolving slice groups can change picture by picture according to the corresponding parameters in the picture parameter set and a slice group change cycle parameter in the slice header. The static slice group patterns include interleaved, checkerboard, rectangular oriented, and freeform. The evolving slice group patterns include horizontal wipe, vertical wipe, box-in, and box-out. The rectangular oriented pattern and the evolving patterns are especially suited for coding of isolated regions and are described more carefully in the following.

For a rectangular oriented slice group pattern, a desired number of rectangles are specified within the picture area. A foreground slice group includes the macroblock locations that are within the corresponding rectangle but excludes the macroblock locations that are already allocated by slice groups specified earlier. A leftover slice group contains the macroblocks that are not covered by the foreground slice groups.

An evolving slice group is specified by indicating the scan order of macroblock locations and the change rate of the size of the slice group in number of macroblocks per picture. Each coded picture is associated with a slice group change cycle parameter (conveyed in the slice header). The change cycle multiplied by the change rate indicates the number of macroblocks in the first slice group. The second slice group contains the rest of the macroblock locations.

In H.264/AVC, in-picture prediction is disabled across slice group boundaries, because slice group boundaries lie in slice boundaries. Therefore each slice group is an isolated region or leftover region.

Each slice group has an identification number within a picture. Encoders can restrict the motion vectors in a way that they only refer to the decoded macroblocks belonging to slice groups having the same identification number as the slice group to be encoded. Encoders should take into account the fact that a range of source samples is needed in fractional pixel interpolation and all the source samples should be within a particular slice group.

The H.264/AVC codec includes a deblocking loop filter. Loop filtering is applied to each 4×4 block boundary, but loop filtering can be turned off by the encoder at slice boundaries. If loop filtering is turned off at slice boundaries, perfect reconstructed pictures at the decoder can be achieved when performing gradual random access. Otherwise, reconstructed pictures may be imperfect in content even after the recovery point.

The recovery point SEI message and the motion constrained slice group set SEI message of the H.264/AVC standard can be used to indicate that some slice groups are coded as isolated regions with restricted motion vectors. Decoders may utilize the information for example to achieve faster random access or to save in processing time by ignoring the leftover region.

A sub-picture concept has been proposed for HEVC e.g. in document JCTVC-I0356<http://phenix.int-evry.fr/jct/doc_end_user/documents/9_Geneva/wg11/JCTVC-I0356-v1.zip>, which is similar to rectangular isolated regions or rectangular motion-constrained slice group sets of h.264/AVC. The sub-picture concept proposed in JCTVC-I0356 is described in the following, while it should be understood that sub-pictures may be defined otherwise similarly but not identically to what is described below. In the sub-picture concept, the picture is partitioned into predefined rectangular regions. Each sub-picture would be processed as an independent picture except that all sub-pictures constituting a picture share the same global information such as SPS, PPS and reference picture sets. Sub-pictures are similar to tiles geometrically. Their properties are as follows: They are LCU-aligned rectangular regions specified at sequence level. Sub-pictures in a picture may be scanned in sub-picture raster scan of the picture. Each sub-picture starts a new slice. If multiple tiles are present in a picture, sub-picture boundaries and tiles boundaries may be aligned. There may be no loop filtering across sub-pictures. There may be no prediction of sample value and motion info outside the sub-picture, and no sample value at a fractional sample position that is derived using one or more sample values outside the sub-picture may be used to inter predict any sample within the sub-picture. If motion vectors point to regions outside of a sub-picture, a padding process defined for picture boundaries may be applied. LCUs are scanned in raster scan order within sub-pictures unless a sub-picture contains more than one tile. Tiles within a sub-picture are scanned in tile raster scan order of the sub-picture. Tiles cannot cross sub-picture boundaries except for the default one tile per picture case. All coding mechanisms that are available at picture level are supported at sub-picture level.

Scalable video coding refers to coding structure where one bitstream can contain multiple representations of the content at different bitrates, resolutions or frame rates. In these cases the receiver can extract the desired representation depending on its characteristics (e.g. resolution that matches best the display device). Alternatively, a server or a network element can extract the portions of the bitstream to be transmitted to the receiver depending on e.g. the network characteristics or processing capabilities of the receiver. A scalable bitstream typically consists of a “base layer” providing the lowest quality video available and one or more enhancement layers that enhance the video quality when received and decoded together with the lower layers. In order to improve coding efficiency for the enhancement layers, the coded representation of that layer may depend on the lower layers. E.g. the motion and mode information of the enhancement layer can be predicted from lower layers. Similarly the pixel data of the lower layers can be used to create prediction for the enhancement layer.

In some scalable video coding schemes, a video signal can be encoded into a base layer and one or more enhancement layers. An enhancement layer may enhance the temporal resolution (i.e., the frame rate), the spatial resolution, or simply the quality of the video content represented by another layer or part thereof. Each layer together with all its dependent layers is one representation of the video signal at a certain spatial resolution, temporal resolution and quality level. In this document, we refer to a scalable layer together with all of its dependent layers as a “scalable layer representation”. The portion of a scalable bitstream corresponding to a scalable layer representation can be extracted and decoded to produce a representation of the original signal at certain fidelity.

Some coding standards allow creation of scalable bit streams. A meaningful decoded representation can be produced by decoding only certain parts of a scalable bit stream. Scalable bit streams can be used for example for rate adaptation of pre-encoded unicast streams in a streaming server and for transmission of a single bit stream to terminals having different capabilities and/or with different network conditions. A list of some other use cases for scalable video coding can be found in the ISO/IEC JTC1 SC29 WG11 (MPEG) output document N5540, “Applications and Requirements for Scalable Video Coding”, the 64^(th) MPEG meeting, Mar. 10 to 14, 2003, Pattaya, Thailand.

In some cases, data in an enhancement layer can be truncated after a certain location, or even at arbitrary positions, where each truncation position may include additional data representing increasingly enhanced visual quality. Such scalability is referred to as fine-grained (granularity) scalability (FGS).

SVC uses an inter-layer prediction mechanism, wherein certain information can be predicted from layers other than the currently reconstructed layer or the next lower layer. Information that could be inter-layer predicted includes intra texture, motion and residual data. Inter-layer motion prediction includes the prediction of block coding mode, header information, etc., wherein motion from the lower layer may be used for prediction of the higher layer. In case of intra coding, a prediction from surrounding macroblocks or from co-located macroblocks of lower layers is possible. These prediction techniques do not employ information from earlier coded access units and hence, are referred to as intra prediction techniques. Furthermore, residual data from lower layers can also be employed for prediction of the current layer.

Scalable video (de)coding may be realized with a concept known as single-loop decoding, where decoded reference pictures are reconstructed only for the highest layer being decoded while pictures at lower layers may not be fully decoded or may be discarded after using them for inter-layer prediction. In single-loop decoding, the decoder performs motion compensation and full picture reconstruction only for the scalable layer desired for playback (called the “desired layer” or the “target layer”), thereby reducing decoding complexity when compared to multi-loop decoding. All of the layers other than the desired layer do not need to be fully decoded because all or part of the coded picture data is not needed for reconstruction of the desired layer. However, lower layers (than the target layer) may be used for inter-layer syntax or parameter prediction, such as inter-layer motion prediction. Additionally or alternatively, lower layers may be used for inter-layer intra prediction and hence intra-coded blocks of lower layers may have to be decoded. Additionally or alternatively, inter-layer residual prediction may be applied, where the residual information of the lower layers may be used for decoding of the target layer and the residual information may need to be decoded or reconstructed. In some coding arrangements, a single decoding loop is needed for decoding of most pictures, while a second decoding loop may be selectively applied to reconstruct so-called base representations (i.e. decoded base layer pictures), which may be needed as prediction references but not for output or display.

SVC allows the use of single-loop decoding. It is enabled by using a constrained intra texture prediction mode, whereby the inter-layer intra texture prediction can be applied to macroblocks (MBs) for which the corresponding block of the base layer is located inside intra-MBs. At the same time, those intra-MBs in the base layer use constrained intra-prediction (e.g., having the syntax element “constrained intra_pred flag” equal to 1). In single-loop decoding, the decoder performs motion compensation and full picture reconstruction only for the scalable layer desired for playback (called the “desired layer” or the “target layer”), thereby greatly reducing decoding complexity. All of the layers other than the desired layer do not need to be fully decoded because all or part of the data of the MBs not used for inter-layer prediction (be it inter-layer intra texture prediction, inter-layer motion prediction or inter-layer residual prediction) is not needed for reconstruction of the desired layer.

A single decoding loop is needed for decoding of most pictures, while a second decoding loop is selectively applied to reconstruct the base representations, which are needed as prediction references but not for output or display, and are reconstructed only for the so called key pictures (for which “store_ref_base_pic_flag” is equal to 1).

FGS was included in some draft versions of the SVC standard, but it was eventually excluded from the final SVC standard. FGS is subsequently discussed in the context of some draft versions of the SVC standard. The scalability provided by those enhancement layers that cannot be truncated is referred to as coarse-grained (granularity) scalability (CGS). It collectively includes the traditional quality (SNR) scalability and spatial scalability. The SVC standard supports the so-called medium-grained scalability (MGS), where quality enhancement pictures are coded similarly to SNR scalable layer pictures but indicated by high-level syntax elements similarly to FGS layer pictures, by having the quality_id syntax element greater than 0.

The scalability structure in the SVC draft may be characterized by three syntax elements: “temporal_id,” “dependency_id” and “quality_id.” The syntax element “temporal_id” is used to indicate the temporal scalability hierarchy or, indirectly, the frame rate. A scalable layer representation comprising pictures of a smaller maximum “temporal_id” value has a smaller frame rate than a scalable layer representation comprising pictures of a greater maximum “temporal_id”. A given temporal layer typically depends on the lower temporal layers (i.e., the temporal layers with smaller “temporal_id” values) but does not depend on any higher temporal layer. The syntax element “dependency_id” is used to indicate the CGS inter-layer coding dependency hierarchy (which, as mentioned earlier, includes both SNR and spatial scalability). At any temporal level location, a picture of a smaller “dependency_id” value may be used for inter-layer prediction for coding of a picture with a greater “dependency_id” value. The syntax element “quality_id” is used to indicate the quality level hierarchy of a FGS or MGS layer. At any temporal location, and with an identical “dependency_id” value, a picture with “quality_id” equal to QL uses the picture with “quality_id” equal to QL-1 for inter-layer prediction. A coded slice with “quality_id” larger than 0 may be coded as either a truncatable FGS slice or a non-truncatable MGS slice.

For simplicity, all the data units (e.g., Network Abstraction Layer units or NAL units in the SVC context) in one access unit having identical value of “dependency_id” are referred to as a dependency unit or a dependency representation. Within one dependency unit, all the data units having identical value of “quality_id” are referred to as a quality unit or layer representation.

A base representation, also known as a decoded base picture, is a decoded picture resulting from decoding the Video Coding Layer (VCL) NAL units of a dependency unit having “quality_id” equal to 0 and for which the “store_ref_base_pic_flag” is set equal to 1. An enhancement representation, also referred to as a decoded picture, results from the regular decoding process in which all the layer representations that are present for the highest dependency representation are decoded.

As mentioned earlier, CGS includes both spatial scalability and SNR scalability. Spatial scalability is initially designed to support representations of video with different resolutions. For each time instance, VCL NAL units are coded in the same access unit and these VCL NAL units can correspond to different resolutions. During the decoding, a low resolution VCL NAL unit provides the motion field and residual which can be optionally inherited by the final decoding and reconstruction of the high resolution picture. When compared to older video compression standards, SVC's spatial scalability has been generalized to enable the base layer to be a cropped and zoomed version of the enhancement layer.

MGS quality layers are indicated with “quality_id” similarly as FGS quality layers. For each dependency unit (with the same “dependency_id”), there is a layer with “quality_id” equal to 0 and there can be other layers with “quality_id” greater than 0. These layers with “quality_id” greater than 0 are either MGS layers or FGS layers, depending on whether the slices are coded as truncatable slices.

In the basic form of FGS enhancement layers, only inter-layer prediction is used. Therefore, FGS enhancement layers can be truncated freely without causing any error propagation in the decoded sequence. However, the basic form of FGS suffers from low compression efficiency. This issue arises because only low-quality pictures are used for inter prediction references. It has therefore been proposed that FGS-enhanced pictures be used as inter prediction references. However, this may cause encoding-decoding mismatch, also referred to as drift, when some FGS data are discarded.

One feature of a draft SVC standard is that the FGS NAL units can be freely dropped or truncated, and a feature of the SVCV standard is that MGS NAL units can be freely dropped (but cannot be truncated) without affecting the conformance of the bitstream. As discussed above, when those FGS or MGS data have been used for inter prediction reference during encoding, dropping or truncation of the data would result in a mismatch between the decoded pictures in the decoder side and in the encoder side. This mismatch is also referred to as drift.

To control drift due to the dropping or truncation of FGS or MGS data, SVC applied the following solution: In a certain dependency unit, a base representation (by decoding only the CGS picture with “quality_id” equal to 0 and all the dependent-on lower layer data) is stored in the decoded picture buffer. When encoding a subsequent dependency unit with the same value of “dependency_id,” all of the NAL units, including FGS or MGS NAL units, use the base representation for inter prediction reference. Consequently, all drift due to dropping or truncation of FGS or MGS NAL units in an earlier access unit is stopped at this access unit. For other dependency units with the same value of “dependency_id,” all of the NAL units use the decoded pictures for inter prediction reference, for high coding efficiency.

Each NAL unit includes in the NAL unit header a syntax element “use_ref_base_pic_flag.” When the value of this element is equal to 1, decoding of the NAL unit uses the base representations of the reference pictures during the inter prediction process. The syntax element “store_ref_base_pic_flag” specifies whether (when equal to 1) or not (when equal to 0) to store the base representation of the current picture for future pictures to use for inter prediction.

NAL units with “quality_id” greater than 0 do not contain syntax elements related to reference picture lists construction and weighted prediction, i.e., the syntax elements “num_ref_active_1x_minus1” (x=0 or 1), the reference picture list reordering syntax table, and the weighted prediction syntax table are not present. Consequently, the MGS or FGS layers have to inherit these syntax elements from the NAL units with “quality_id” equal to 0 of the same dependency unit when needed.

In SVC, a reference picture list consists of either only base representations (when “use_ref_base_pic_flag” is equal to 1) or only decoded pictures not marked as “base representation” (when “use_ref_base_pic_flag” is equal to 0), but never both at the same time.

A scalable nesting SEI message has been specified in SVC. The scalable nesting SEI message provides a mechanism for associating SEI messages with subsets of a bitstream, such as indicated dependency representations or other scalable layers. A scalable nesting SEI message contains one or more SEI messages that are not scalable nesting SEI messages themselves. An SEI message contained in a scalable nesting SEI message is referred to as a nested SEI message. An SEI message not contained in a scalable nesting SEI message is referred to as a non-nested SEI message. A similar SEI message than the scalable nesting SEI message has been specified in MVC for indicating which views the nested SEI messages apply to. Another similar SEI message than the scalable nesting SEI message has been specified in the multiview and depth extension of H.264/AVC (also referred to as MVC+D) to specify which texture and/or depth views the nested SEI messages apply to. H.265/HEVC also includes a similar scalable nesting SEI message.

A scalable video codec for quality scalability (also known as Signal-to-Noise or SNR) and/or spatial scalability may be implemented as follows. For a base layer, a conventional non-scalable video encoder and decoder are used. The reconstructed/decoded pictures of the base layer are included in the reference picture buffer for an enhancement layer. In H.264/AVC, HEVC, and similar codecs using reference picture list(s) for inter prediction, the base layer decoded pictures may be inserted into a reference picture list(s) for coding/decoding of an enhancement layer picture similarly to the decoded reference pictures of the enhancement layer. Consequently, the encoder may choose a base-layer reference picture as inter prediction reference and indicate its use typically with a reference picture index in the coded bitstream. The decoder decodes from the bitstream, for example from a reference picture index, that a base-layer picture is used as inter prediction reference for the enhancement layer. When a decoded base-layer picture is used as prediction reference for an enhancement layer, it is referred to as an inter-layer reference picture.

In addition to quality scalability following scalability modes exist:

Spatial scalability: Base layer pictures are coded at a lower resolution than enhancement layer pictures.

Bit-depth scalability: Base layer pictures are coded at lower bit-depth (e.g. 8 bits) than enhancement layer pictures (e.g. 10 or 12 bits).

Chroma format scalability: Base layer pictures provide lower fidelity in chroma (e.g. coded in 4:2:0 chroma format) than enhancement layer pictures (e.g. 4:4:4 format).

Color gamut scalability, where the enhancement layer pictures have a richer/broader color representation range than that of the base layer pictures—for example the enhancement layer may have UHDTV (ITU-R BT.2020) color gamut and the base layer may have the ITU-R BT.709 color gamut.

View scalability, where different layers represent different views of multiview video.

Depth scalability, where certain layers may represent regular color video content and others may represent ranging information, disparity, depth, or alike.

Auxiliary picture scalability, where certain layers may represent auxiliary video content such as alpha planes, which may be used for example to indicate transparency or opacity information or for chroma keying.

In all of the above scalability cases, base layer information could be used to code enhancement layer to minimize the additional bitrate overhead.

In the following, the operation of the inter-layer processing block 450 according to some embodiments is disclosed in more detail with reference to FIG. 4b . In this example it is assumed that the source for the inter-layer sample prediction and/or inter-layer motion information prediction is the base layer and a target layer i.e. the layer for which the prediction is performed is an enhancement layer. In some embodiments the source layer may be an enhancement layer (e.g. a first enhancement layer) and the target layer may be another enhancement layer at a higher level (e.g. a second enhancement layer).

The second encoder section 502 may have determined which picture of the base layer may be used as a reference picture for the current block of the enhancement layer. The second encoder section 502 may then indicate the picture to the first encoder section 500 which may provide the sample values of the requested reconstructed reference layer image from the reference frame memory 318, or the second encoder section 502 may directly obtain the reconstructed reference layer image from the reference frame memory 318.

The inter-layer processing block 450 receives 452 sample values of a reconstructed reference layer image from the reference frame memory 318. The reconstructed reference layer image may be, for example, a base layer image having a smaller resolution and/or smaller bit-depth than images of the enhancement layer. The received reconstructed reference layer image may be provided to an upsampling filter 452 to increase the spatial resolution of the reference layer image. The upsampling filter 452 may perform an upsampling filtering operation that also increases the bit-depth of the received reconstructed reference layer image and to obtain an upsampled image, which has higher resolution and higher bit-depth than the base layer image. In some embodiments the upsampling filter 452 multiplies samples of the reconstructed reference layer image with filter coefficients. The upsampled image may then be modified e.g. by adding a rounding offset 456 to the values of the upsampled image. The rounding offset may depend on accuracy of filter coefficients and/or the difference between bit-depth of the two layers (e.g. the enhancement and the base layer). The bit-depth of the result may be shifted 458 to correspond with the bit-depth of the target layer, which in this example is the (first) enhancement layer. The shift may depend on accuracy of filter coefficients and/or the difference between the target layer and the source layer bit-depth. The bit-depth shifted values may further be provided to a value range limiter 460 which may modify the reconstructed reference layer image such that the values of the reconstructed reference layer image are within a predetermined value range. For example, the value range may be from 0 to the maximum value of the base layer bit-depth. In some embodiments the bit-depth of the target layer may be 10 bits and base layer may be 8 bits, but also other bit-depths may be implemented. The output 462 from the inter layer processing block 460 may be provided to the predictor selector 406 of the enhancement layer.

For completeness a suitable decoder is hereafter described. However, some decoders may not be able to process enhancement layer data wherein they may not be able to decode all received images.

At the decoder side similar operations may be performed to reconstruct the image blocks. FIG. 5 shows a block diagram of a video decoder 550 suitable for employing embodiments of the invention. In this embodiment the video decoder 550 comprises a first decoder section 552 for base view components and a second decoder section 554 for non-base view components. Block 556 illustrates a demultiplexer for delivering information regarding base view components to the first decoder section 552 and for delivering information regarding non-base view components to the second decoder section 554. The decoder shows an entropy decoder 700, 800 which performs an entropy decoding (E⁻¹) on the received signal. The entropy decoder thus performs the inverse operation to the entropy encoder 330, 430 of the encoder described above. The entropy decoder 700, 800 outputs the results of the entropy decoding to a prediction error decoder 701, 801 and pixel predictor 704, 804. Reference P′_(n) stands for a predicted representation of an image block. Reference D′_(n) stands for a reconstructed prediction error signal. Blocks 705, 805 illustrate preliminary reconstructed images or image blocks (I′_(n)). Reference R′_(n) stands for a final reconstructed image or image block. Blocks 703, 803 illustrate inverse transform (T⁻¹). Blocks 702, 802 illustrate inverse quantization (Q⁻¹). Blocks 706, 806 illustrate a reference frame memory (RFM). Blocks 707, 807 illustrate prediction (P) (either inter prediction or intra prediction). Blocks 708, 808 illustrate filtering (F). Blocks 709, 809 may be used to combine decoded prediction error information with predicted base view/non-base view components to obtain the preliminary reconstructed images (I′_(n)). Preliminary reconstructed and filtered base view images may be output 710 from the first decoder section 552 and preliminary reconstructed and filtered base view images may be output 810 from the second decoder section 554.

The pixel predictor 704, 804 receives the output of the entropy decoder 700, 800. The output of the entropy decoder 700, 800 may include an indication on the prediction mode used in encoding the current block. A predictor selector 707, 807 within the pixel predictor 704, 804 may determine that the current block to be decoded is an enhancement layer block. Hence, the predictor selector 707, 807 may select to use information from a corresponding block on another layer such as the base layer to filter the base layer prediction block while decoding the current enhancement layer block. An indication that the base layer prediction block has been filtered before using in the enhancement layer prediction by the encoder may have been received by the decoder wherein the pixel predictor 704, 804 may use the indication to provide the reconstructed base layer block values to the filter 708, 808 and to determine which kind of filter has been used, e.g. the SAO filter and/or the adaptive loop filter, or there may be other ways to determine whether or not the modified decoding mode should be used.

The predictor selector may output a predicted representation of an image block P′_(n) to a first combiner 709. The predicted representation of the image block is used in conjunction with the reconstructed prediction error signal D′_(n) to generate a preliminary reconstructed image I′_(n). The preliminary reconstructed image may be used in the predictor 704, 804 or may be passed to a filter 708, 808. The filter applies a filtering which outputs a final reconstructed signal R′_(n). The final reconstructed signal R′_(n) may be stored in a reference frame memory 706, 806, the reference frame memory 706, 806 further being connected to the predictor 707, 807 for prediction operations.

The prediction error decoder 702, 802 receives the output of the entropy decoder 700, 800. A dequantizer 702, 802 of the prediction error decoder 702, 802 may dequantize the output of the entropy decoder 700, 800 and the inverse transform block 703, 803 may perform an inverse transform operation to the dequantized signal output by the dequantizer 702, 802. The output of the entropy decoder 700, 800 may also indicate that prediction error signal is not to be applied and in this case the prediction error decoder produces an all zero output signal.

It should be understood that for various blocks in FIG. 5 inter-layer prediction may be applied, even if it is not illustrated in FIG. 5. Inter-layer prediction may include sample prediction and/or syntax/parameter prediction. For example, a reference picture from one decoder section (e.g. RFM 706) may be used for sample prediction of the other decoder section (e.g. block 807). In another example, syntax elements or parameters from one decoder section (e.g. filter parameters from block 708) may be used for syntax/parameter prediction of the other decoder section (e.g. block 808).

FIG. 5b shows a simplified block diagram of an example embodiment of an inter-layer processing block 580 at a decoder. The inter-layer processing block 580 of the decoder may be similar to the inter-layer processing block 450 of the encoder, described earlier in this application and depicted in FIG. 4b . The second decoder section 554 may have determined which picture of the base layer may be used as a reference picture for the current block of the enhancement layer. The second decoder section 554 may then indicate the picture to the first decoder section 552 which may provide the sample values of the requested decoded reference layer image from the reference frame memory 706, or the second decoder section 554 may directly obtain the decoded reference layer image from the reference frame memory 706, or the second decoder section 554 may receive from a bitstream an indication of the reference layer image and use the indication to obtain the decoded reference layer image from the reference frame memory 706.

The inter-layer processing block 580 may receive sample values of a decoded reference layer image from the reference frame memory 706. The decoded reference layer image may be, for example, a base layer image having a smaller resolution and/or smaller bit-depth than images of the enhancement layer. The decoded reference layer image may be provided to an upsampling filter 582 to increase the spatial resolution of the reference layer image. The upsampling filter 582 may perform an upsampling filtering operation that also increases the bit-depth of the decoded reference layer image and to obtain an upsampled image, which has higher resolution and higher bit-depth than the base layer image. In some embodiments the upsampling filter 582 multiplies samples of the decoded reference layer image with filter coefficients. The upsampled image may then be modified e.g. by adding a rounding offset 586 to the values of the upsampled image. The rounding offset may depend on accuracy of filter coefficients and/or the difference between bit-depth of the two layers (e.g. the enhancement and the base layer). The bit-depth of the result may be shifted 588 to correspond with the bit-depth of the target layer, which in this example is the (first) enhancement layer. The shift may depend on accuracy of filter coefficients and/or the difference between the target layer and the source layer bit-depth. The bit-depth shifted values may further be provided to a value range limiter 590 which may modify the decoded reference layer image such that the values of the decoded reference layer image are within a predetermined value range. For example, the value range may be from 0 to the maximum value of the base layer bit-depth. In some embodiments the bit-depth of the target layer may be 10 bits and base layer may be 8 bits, but also other bit-depths may be implemented. The output 592 from the inter layer processing block 580 may be provided to the predictor selector 807 of the enhancement layer.

In the following some further details of the operation of the resampling process for inter layer reference pictures in the inter-layer processing block 450, 580 according to some embodiments are provided. An input to this process may be a cropped decoded reference layer picture rlPic 452, 582 and the output of this process may be the resampled reference layer picture rsPic 462, 592.

Variables ScaledW (scaled width) and ScaledH (scaled height) may be set equal to the width and height of the current picture in units of luma samples, respectively. The variable rsPicSample specifies the resampled sample values of the luma and chroma components of the reference picture rsPic. The variable rsPicMotion specifies the resampled motion field of the reference picture rsPic.

The variables PicWRL and PicHRL may be set equal to the width and height of the cropped (de)coded reference layer picture rlPic in units of luma samples, respectively. The variable rlPicSample specifies the sample values of the luma and chroma components of the cropped (de)coded reference layer picture rlPic. The variable rlPicMotion specifies the motion field of rlPic.

The following steps may be applied to derive the resampled inter layer reference picture rsPic. If the width ScaledW of the current picture in units of luma samples is equal to the width PicWRL of the cropped decoded reference layer picture and if the height ScaledH of the current picture in units of luma samples is equal to the height PicHRL of the cropped decoded reference layer picture, then the resampled sample values rsPicSample of the luma and chroma components are set equal to the sample values rlPicSample of the luma and chroma components of the cropped decoded reference layer picture rlPic.

When the parameter sps_inter_layer_mfm_enable_flag is equal to 1, the motion field of the resampled inter layer reference picture rsPic is set to the motion field of the cropped (de)coded reference layer picture rlPic (rsPicMotion=rlPicMotion). Otherwise, the resampled inter layer reference picture rsPic may be derived as follows. The picture sample resampling process may be invoked with the sample values of rlPicSample bit-depth of base-layer samples, nBase and bit-depth of enhancement-layer samples, nEnh, as input, and with the resampled sample values of rsPicSample as output. When the parameter sps_inter_layer_mfm_enable_flag is equal to 1, the picture motion field resampling process may be invoked with the compressed motion field of rlPicMotion as input, and with the resampled motion field of rsPicMotion as output.

An example of luma sample interpolation process is described in the following. Inputs to the luma sample interpolation process are the luma reference sample array rlPicSample_(L); a luma sample location (xP, yP) relative to the top-left luma sample of the current picture; bit-depth of base-layer samples, nBase; and bit-depth of enhancement-layer samples, nEnh. Output of this process is a interpolated luma sample value intLumaSample.

The Table 1 specifies an example of an 8-tap filter coefficients f_(L)[p, x] with phase p=0 . . . 15 and x=0 . . . 7 which may be used for the luma resampling process.

TABLE 1 interpolation filter coefficients phase p f_(L)[p, 0] f_(L)[p, 1] f_(L)[p, 2] f_(L)[p, 3] f_(L)[p, 4] f_(L)[p, 5] f_(L)[p, 6] f_(L)[p, 7] 0 0 0 0 64 0 0 0 0 1 n/a n/a n/a n/a n/a n/a n/a n/a 2 n/a n/a n/a n/a n/a n/a n/a n/a 3 n/a n/a n/a n/a n/a n/a n/a n/a 4 n/a n/a n/a n/a n/a n/a n/a n/a 5 −1 4 −11 52 26 −8 3 −1 6 n/a n/a n/a n/a n/a n/a n/a n/a 7 n/a n/a n/a n/a n/a n/a n/a n/a 8 −1 4 −11 40 40 −11 4 −1 9 n/a n/a n/a n/a n/a n/a n/a n/a 10 n/a n/a n/a n/a n/a n/a n/a n/a 11 −1 3 −8 26 52 −11 4 −1 12 n/a n/a n/a n/a n/a n/a n/a n/a 13 n/a n/a n/a n/a n/a n/a n/a n/a 14 n/a n/a n/a n/a n/a n/a n/a n/a 15 n/a n/a n/a n/a n/a n/a n/a n/a

The value of the interpolated luma sample IntLumaSample may be derived by applying the following processes. The derivation process for reference layer sample location used in resampling may be invoked with cIdx equal to 0 and luma sample location (xP, yP) given as the inputs and (xRef16, yRef16) in units of 1/16-th sample as output. The variables xRef and xPhase may be derived as follows:

-   -   xRef=(xRef16>>4)     -   xPhase=(xRef16) % 16

The variables yRef and yPhase may be derived as follows:

-   -   yRef=(yRef16>>4)     -   yPhase=(yRef16) % 16

The sample value tempArray[n] with n=0 . . . 7, may be derived as follows:

-   -   yPosRL=Clip3(0, RefLayerPicHeightInSamplesL−1, yRef+n−1)     -   refW=RefLayerPicWidthInSamplesL     -   tempArray[n]=     -   f_(L)[xPhase, 0]*rlPicSample_(L)[Clip3(0, refW−1, xRef−3),         yPosRL]+         -   f_(L)[xPhase, 1]*rlPicSample_(L)[Clip3(0, refW−1, xRef−2),             yPosR L]+         -   f_(L)[xPhase, 2]*rlPicSample_(L)[Clip3(0, refW−1, xRef−1),             yPosR L]+         -   f_(L)[xPhase, 3]*rlPicSample_(L)[Clip3(0, refW−1, xRef),             yPosR L]+         -   f_(L)[xPhase, 4]*rlPicSample_(L)[Clip3(0, refW−1, xRef+1),             yPosR L]+         -   f_(L)[xPhase, 5]*rlPicSample_(L)[Clip3(0, refW−1, xRef+2),             yPosR L]+         -   f_(L)[xPhase, 6]*rlPicSample_(L)[Clip3(0, refW−1, xRef+3),             yPosR L]+         -   f_(L)[xPhase, 7]*rlPicSample_(L)[Clip3(0, refW−1, xRef+4),             yPosR L]

The interpolated luma sample value intLumaSample may be derived as follows:

-   -   intLumaSample=(f_(L)[yPhase, 0]*tempArray [0]+         -   f_(L)[yPhase, 1]*tempArray [1]+         -   f_(L)[yPhase, 2]*tempArray [2]+         -   f_(L)[yPhase, 3]*tempArray [3]+         -   f_(L)[yPhase, 4]*tempArray [4]+         -   f_(L)[yPhase, 5]*tempArray [5]+         -   f_(L)[yPhase, 6]*tempArray [6]+         -   f_(L)[yPhase, 7]*tempArray             [7]+(1<<(12+nBase−nEnh−1))>>(12+nBase−nEnh)     -   intLumaSample=Clip3(0, (1<<nEnh)−1, intLumaSample)

As shown in the above equation, the bit-depth of the samples are shifted by (12+nBase−nEnh) where the shift depends on number 12, which is derived from the accuracy of filter coefficients and the bit-depth of base and enhancement layer. In addition, the rounding offset is 1<<(12+nBase−nEnh−1) where the rounding offset also depends on the filter accuracy and the bit-depth of base and enhancement layer pictures.

In the above, some embodiments have been described with reference to an enhancement layer and a base layer. It needs to be understood that the base layer may as well be any other layer as long as it is a reference layer for the enhancement layer. It also needs to be understood that term enhancement layer may indicate any non-base layer and need not indicate an enhancement of picture or video quality of the enhancement layer when compared to the picture/video quality of the base/reference layer. It also needs to be understood that the encoder may generate more than two layers into a bitstream and the decoder may decode more than two layers from the bitstream. Embodiments could be realized with any pair of an enhancement layer and its reference layer. Likewise, many embodiments could be realized with consideration of more than two layers.

Embodiments of the present invention may be implemented in software, hardware, application logic or a combination of software, hardware and application logic. In an example embodiment, the application logic, software or an instruction set is maintained on any one of various conventional computer-readable media. In the context of this document, a “computer-readable medium” may be any media or means that can contain, store, communicate, propagate or transport the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer, with one example of a computer described and depicted in FIGS. 1 and 2. A computer-readable medium may comprise a computer-readable storage medium that may be any media or means that can contain or store the instructions for use by or in connection with an instruction execution system, apparatus, or device, such as a computer.

If desired, the different functions discussed herein may be performed in a different order and/or concurrently with each other. Furthermore, if desired, one or more of the above-described functions may be optional or may be combined.

Although the above examples describe embodiments of the invention operating within a codec within an electronic device, it would be appreciated that the invention as described below may be implemented as part of any video codec. Thus, for example, embodiments of the invention may be implemented in a video codec which may implement video coding over fixed or wired communication paths.

Thus, user equipment may comprise a video codec such as those described in embodiments of the invention above. It shall be appreciated that the term user equipment is intended to cover any suitable type of wireless user equipment, such as mobile telephones, portable data processing devices or portable web browsers.

Furthermore elements of a public land mobile network (PLMN) may also comprise video codecs as described above.

In general, the various embodiments of the invention may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. For example, some aspects may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatuses, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

The embodiments of this invention may be implemented by computer software executable by a data processor of the mobile device, such as in the processor entity, or by hardware, or by a combination of software and hardware. Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD.

The various embodiments of the invention can be implemented with the help of computer program code that resides in a memory and causes the relevant apparatuses to carry out the invention. For example, a terminal device may comprise circuitry and electronics for handling, receiving and transmitting data, computer program code in a memory, and a processor that, when running the computer program code, causes the terminal device to carry out the features of an embodiment. Yet further, a network device may comprise circuitry and electronics for handling, receiving and transmitting data, computer program code in a memory, and a processor that, when running the computer program code, causes the network device to carry out the features of an embodiment.

The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on multi-core processor architecture, as non-limiting examples.

Embodiments of the inventions may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

Programs, such as those provided by Synopsys Inc., of Mountain View, Calif. and Cadence Design, of San Jose, Calif. automatically route conductors and locate components on a semiconductor chip using well established rules of design as well as libraries of pre-stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or “fab” for fabrication.

The foregoing description has provided by way of exemplary and non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention.

In the following some examples will be provided.

According to a first example there is provided a method comprising:

obtaining information on bit-depth of a reference block of an image on a first layer;

obtaining information on bit-depth of a current block of an image on a second layer;

using the obtained information to select one or more resampling filter parameters for filtering at least a part of a reference layer image to be used in inter-layer prediction.

In some embodiments the method further comprises:

obtaining information on a spatial resolution of the reference block and the current block; and

using the obtained information on the spatial resolution together with the information on the bit-depth in the selection of one or more resampling filter parameters.

In some embodiments of the method the filtering comprises:

multiplying the samples of a reconstructed image of the first layer with filter coefficients and summing the result;

adding a rounding offset to the resulting sum; and

adjusting the bit-depth of the result of the adding operation.

In some embodiments of the method the obtaining information on bit depth of a reference block of an image on a first layer and obtaining information on bit-depth of a current block of an image on a second layer comprises reading one or more syntax elements from a bitstream.

In some embodiments the method comprises:

determining the rounding offset on the basis of one or more of the accuracy of filter coefficients and the difference between bit-depths of the enhancement layer and the base layer.

In some embodiments of the method the adjusting comprises:

shifting the result of the adding operation to the bit-depth of the enhancement layer.

In some embodiments the method further comprises:

clipping the filtered values to limit the values within a predetermined value range.

In some embodiments the method further comprises using the filtered reference layer image in inter-layer prediction to encode a current block.

In some embodiments the method further comprises using the filtered reference layer image in inter-layer prediction to decode a current block.

According to a second example there is provided an apparatus comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to:

obtain information on bit-depth of a reference block of an image on a first layer;

obtain information on bit-depth of a current block of an image on a second layer;

use the obtained information to select one or more resampling filter parameters for filtering at least a part of a reference layer image to be used in inter-layer prediction.

In some embodiments of the apparatus said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to:

obtain information on a spatial resolution of the reference block and the current block; and;

use the obtained information on the spatial resolution together with the information on the bit-depth in the selection of one or more resampling filter parameters.

In some embodiments of the apparatus said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to:

multiply the samples of a reconstructed image of the first layer with filter coefficients and summing the result;

add a rounding offset to the resulting sum; and

adjust the bit-depth of the result of the adding operation.

In some embodiments of the apparatus said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to obtain information on bit depth of a reference block of an image on a first layer and to obtain information on bit-depth of a current block of an image on a second layer by reading one or more syntax elements from a bitstream.

In some embodiments of the apparatus said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to:

determine the rounding offset on the basis of one or more of the accuracy of filter coefficients and the difference between bit-depths of the enhancement layer and the base layer.

In some embodiments of the apparatus said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to:

shift the result of the adding operation to the bit-depth of the enhancement layer.

In some embodiments of the apparatus said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to:

clip the filtered values to limit the values within a predetermined value range.

In some embodiments of the apparatus said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to:

use the filtered reference layer image in inter-layer prediction to encode a current block.

In some embodiments of the apparatus said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to:

use the filtered reference layer image in inter-layer prediction to decode a current block.

According to a third example there is provided a computer program product including one or more sequences of one or more instructions which, when executed by one or more processors, cause an apparatus to at least perform the following:

obtain information on bit-depth of a reference block of an image on a first layer;

obtain information on bit-depth of a current block of an image on a second layer;

use the obtained information to select one or more resampling filter parameters for filtering at least a part of a reference layer image to be used in inter-layer prediction.

In some embodiments the computer program product further comprises one or more sequences of one or more instructions which, when executed by one or more processors, cause the apparatus to:

obtain information on a spatial resolution of the reference block and the current block; and;

use the obtained information on the spatial resolution together with the information on the bit-depth in the selection of one or more resampling filter parameters.

In some embodiments the computer program product further comprises one or more sequences of one or more instructions which, when executed by one or more processors, cause the apparatus to:

-   -   multiply the samples of a reconstructed image of the first layer         with filter coefficients and summing the result;     -   add a rounding offset to the resulting sum; and     -   adjust the bit-depth of the result of the adding operation.

In some embodiments the computer program product further comprises one or more sequences of one or more instructions which, when executed by said at least one processor, further causes the apparatus to obtain information on bit depth of a reference block of an image on a first layer and to obtain information on bit-depth of a current block of an image on a second layer by reading one or more syntax elements from a bitstream.

In some embodiments the computer program product further comprises one or more sequences of one or more instructions which, when executed by one or more processors, cause the apparatus to:

determine the rounding offset on the basis of one or more of the accuracy of filter coefficients and the difference between bit-depths of the enhancement layer and the base layer.

In some embodiments the computer program product further comprises one or more sequences of one or more instructions which, when executed by one or more processors, cause the apparatus to:

shift the result of the adding operation to the bit-depth of the enhancement layer.

In some embodiments the computer program product further comprises one or more sequences of one or more instructions which, when executed by one or more processors, cause the apparatus to use the filtered reference layer image in inter-layer prediction to encode a current block.

In some embodiments the computer program product further comprises one or more sequences of one or more instructions which, when executed by one or more processors, cause the apparatus to use the filtered reference layer image in inter-layer prediction to decode a current block.

According to a fourth example there is provided an apparatus comprising:

means for obtaining information on bit-depth of a reference block of an image on a first layer;

means for obtaining information on bit-depth of a current block of an image on a second layer;

means for using the obtained information to select one or more resampling filter parameters for filtering at least a part of a reference layer image to be used in inter-layer prediction.

In some embodiments the apparatus comprises:

means for obtaining information on a spatial resolution of the reference block and the current block; and

means for using the obtained information on the spatial resolution together with the information on the bit-depth in the selection of one or more resampling filter parameters.

In some embodiments the apparatus comprises:

means for multiplying the samples of a reconstructed image of the first layer with filter coefficients and for summing the result;

means for adding a rounding offset to the resulting sum; and

means for adjusting the bit-depth of the result of the adding operation.

In some embodiments of the apparatus the means for obtaining information on bit depth of a reference block of an image on a first layer and means for obtaining information on bit-depth of a current block of an image on a second layer comprise means for reading one or more syntax elements from a bitstream.

In some embodiments the apparatus comprises:

means for determining the rounding offset on the basis of one or more of the accuracy of filter coefficients and the difference between bit-depths of the enhancement layer and the base layer.

In some embodiments the apparatus comprises:

means for shifting the result of the adding operation to the bit-depth of the enhancement layer.

In some embodiments the apparatus comprises:

means for clipping the filtered values to limit the values within a predetermined value range.

In some embodiments the apparatus comprises means for using the filtered reference layer image in inter-layer prediction to encode a current block.

In some embodiments the apparatus comprises means for using the filtered reference layer image in inter-layer prediction to decode a current block.

According to a fifth example, there is provided an apparatus configured to perform the method according to the first example.

According to a sixth example, there is provided a decoder configured to perform the method according to the first example.

According to a seventh example, there is provided an encoder configured to perform the method according to the first example. 

The invention claimed is:
 1. A method comprising: obtaining information on bit-depth and spatial resolution of a reference block of a reconstructed reference image of a base layer; obtaining information on bit-depth and spatial resolution of a current block of an enhancement layer image, wherein the reconstructed reference image has a smaller bit depth and smaller resolution than the enhancement layer image; and using the obtained information on bit depth to scale one or more interpolation filter coefficients of an interpolation filter for filtering at least a part of the reconstructed reference image to be used in inter-layer prediction, wherein the filtering comprises multiplying samples of the reconstructed reference image of the base layer with filter coefficients, summing the result, adding a rounding offset to the resulting sum and adjusting a bit-depth of the result of the adding operation by shifting the result of the adding operation to the bit-depth of the enhancement layer image.
 2. The method according to claim 1, further comprising: determining the rounding offset on the basis of one or more of an accuracy of filter coefficients and the difference between bit-depths of the enhancement layer image and the base layer.
 3. The method according to claim 1, wherein the obtaining information on bit depth of a reference block of the reconstructed reference image of the base layer and obtaining information on bit-depth of the current block of the enhancement layer image comprises reading one or more syntax elements from a bitstream.
 4. The method according to claim 1, further comprising: clipping values resulting from the filtering to limit the values within a predetermined value range.
 5. The method according to claim 1, further comprising: using the filtered reconstructed reference image of the base layer in inter-layer prediction to encode or decode a current block.
 6. An apparatus comprising at least one processor and at least one memory including computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus to: obtain information on bit-depth and spatial resolution of a reference block of a reconstructed reference image of a base layer; obtain information on bit-depth and spatial resolution of a current block of an enhancement layer image, wherein the reconstructed reference image has a smaller bit depth and smaller resolution than the enhancement layer image; and use the obtained information on bit depth to scale one or more interpolation filter coefficients of an interpolation filter for filtering at least a part of the reconstructed reference image to be used in inter-layer prediction, wherein the filtering comprises multiplying samples of the reconstructed reference image of the base layer with filter coefficients, summing the result, adding a rounding offset to the resulting sum and adjusting a bit-depth of the result of the adding operation by shifting the result of the adding operation to the bit-depth of the enhancement layer image.
 7. The apparatus according to claim 6, said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to: determine the rounding offset on the basis of one or more of an accuracy of filter coefficients and the difference between bit-depths of the enhancement layer image and the base layer.
 8. The apparatus according to claim 6, said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to obtain information on bit depth of a reference block of the reconstructed reference image of the base layer and obtaining information on bit-depth of current block of the enhancement layer image comprises reading one or more syntax elements from a bitstream.
 9. The apparatus according to claim 6, said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to: clip values resulting from the filtering to limit the values within a predetermined value range.
 10. The apparatus according to claim 6, said at least one memory stored with code thereon, which when executed by said at least one processor, further causes the apparatus to: use the filtered reconstructed reference image of the base layer in inter-layer prediction to encode or decode a current block.
 11. A computer program product comprising at least one non-transitory computer readable storage medium having one or more sequences of one or more instructions which, when executed by one or more processors, cause an apparatus to at least perform the following: obtain information on bit-depth and spatial resolution of a reference block of a reconstructed reference image of a base layer; obtain information on bit-depth and spatial resolution of a current block of an enhancement layer image, wherein the reconstructed reference image has a smaller bit depth and smaller resolution than the enhancement layer image; and use the obtained information on bit depth to scale one or more interpolation filter coefficients of an interpolation filter for filtering at least a part of the reconstructed reference image to be used in inter-layer prediction, wherein the filtering comprises multiplying samples of the reconstructed reference image of the base layer with filter coefficients, summing the result, adding a rounding offset to the resulting sum and adjusting a bit-depth of the result of the adding operation by shifting the result of the adding operation to the bit-depth of the enhancement layer image.
 12. The computer program product according to claim 11, which, when executed by one or more processors, further cause an apparatus to at least perform the following: determining the rounding offset on the basis of one or more of an accuracy of filter coefficients and the difference between bit-depths of the enhancement layer image and the base layer.
 13. The method according to claim 1, wherein the interpolation filter performs a spatial upsampling filtering operation that also increases the bit depth.
 14. The apparatus according to claim 6, wherein the interpolation filter performs a spatial upsampling filtering operation that also increases the bit depth.
 15. The computer program product according to claim 11, wherein the interpolation filter performs a spatial upsampling filtering operation that also increases the bit depth. 